More and more bridges are becoming inseparable from people's lives, gradually merging into cultural relics. Their protection has actually been underway for many years. Since 1986, the project has spanned over thirty years to develop a complete set of theories, methods, and technologies (referred to as SECRET technology) for the full life cycle tracking and monitoring, as well as holistic conservation of cultural relic steel bridges, which have undergone engineering verification: consisting of three parts: existing steel bridge safety prediction and control theory, engineering evaluation method for residual service life consumption, and digital twin technology for performance evolution. Taking the only remaining three-hinged steel arch bridge in China, Ningbo Ling Bridge, as a demonstration, this technology has overcome technical difficulties such as state tracking throughout the life cycle, bidirectional control of virtual and real twins during management and maintenance, criteria for determining the end of service life, and loss of historical technical data. It has also derived two unique holistic conservation methods and technologies, achieving the dual goals of cultural relic protection and effective utilization. In July 2016, Lingqiao Bridge was reborn and hailed by experts as a successful example of the overall protection of cultural relics bridges throughout their entire lifespan, unseen in history. The main technological innovations include: 1) Establishing an engineering evaluation method for the remaining service life consumption of the old steel bridge based on digital twin technology. Relying on the progress of digital twin technology and research methods, a theoretical and methodological system for evaluating the remaining service life based on fracture mechanics has gradually formed and improved. This achieved bidirectional digital flow and virtual-real control in aspects such as inspection evaluation at various periods, traffic management, and performance prediction, with twinning errors not exceeding 5%. The recommended regular inspection interval of about 7 years was adopted by the maintenance standards. Digital twin models were used to timely predict the dangerous state of the cultural relic steel arch bridge. 2) Proposing gene extraction, recombination, and protection criteria applicable to cultural relic bridges. A cultural heritage bridge evaluation method based on the cultural relic gene metadata model and social system value has been established, and protection guidelines have been formulated accordingly, achieving a balance and unity between cultural relic conservation and traffic utilization. With up to 80% of the original materials retained, the traffic function of Ling Bridge was upgraded from an unsafe bridge before major repairs to a Class B city-passable bridge, and its service life was extended by at least 40 years. 3. A unique segment disassembly and reinforcement method and construction process control technology were proposed for three-hinged steel arch bridges. Based on experiments and theoretical analysis, an arch rib lining composite reinforcement method was proposed, which allowed the old structure to share up to 60% of the total load. A new non-destructive/minimally destructive riveting disassembly process has been developed. The trial production of a large steel plate composite structure mechanical correction special device with a length of 20m solved the difficult problem of correcting severely skewed and deformed arch ribs. Through precise control of each segment, the accuracy of restoring the arch axis line was not more than 2mm per meter. 4. A design method and construction process for reinforcing bridge abutment foundations buried deep under unknown conditions with ultra-large diameter piles have been established. In response to factors such as missing foundation drawings, disturbance to the original foundation during construction, investment constraints, cultural relic protection restrictions, and concrete fragmentation at the original arch hinge, a co-action theory was proposed to add ultra-large diameter pile foundations below the front arch hinge, forming a complete set of processes. This ensures that the force on the original abutment is controlled within its capacity before reinforcement, accounting for 50% of the combined abutment's load-bearing capacity. An advanced hole-forming control process was adopted, with a cleaning coefficient lower than 0.6, to ensure zero disturbance to the original state of the inclined wooden pile foundation during the close-range construction period of the ultra-large diameter cast-in-place piles. The project research resulted in 10 authorized invention patents, 64 SCI/EI papers published, participation in 4 specifications, publication of 3 monographs, and training Wang Chunsheng to win the National Excellent Doctoral Dissertation Award. The achievements have been successfully applied to the Wai Baidu Bridge, Nanjing Yangtze River Bridge, The liberation bridge and other twenty-odd cultural relic steel bridges or old steel bridges have generated an additional direct economic benefit of about 3.45 billion yuan in the past three years, and their application prospects will be even broader. According to the verification and acceptance of authoritative institutions, the project has achieved independent innovation in theory, method, and technology, as well as international surpassing, reaching an overall international leading level.