This invention relates to a high boron content boron-modified phenolic resin and its preparation method. The invention adopts a two-step process, where a specific proportion of boric acid reacts with phenol first to form a borate ester, which is then used to react with a specific molecular weight of polyoxymethylene to obtain the high boron content boron-modified phenolic resin. The reaction process, by reasonably controlling the reaction temperature, time, and other conditions, and using modern instrumental analysis methods for monitoring, can yield high boron content boron-modified phenolic resins with different boron contents. The provided preparation method is simple and easy to implement, characterized by controllability and quantification; the resulting boron-modified phenolic resin has a high boron content and exhibits excellent thermal properties, mechanical properties, friction properties, and ablation resistance, making it widely applicable in many fields such as high-temperature brake friction materials, ablation-resistant materials, special structural materials, and thermal protection materials.