This invention belongs to the field of transportation technology and specifically relates to a method for testing the longitudinal force on seamless line track structure rails. The invention is based on the principle that the lateral vibration damping of seamless line steel rails increases with the increase in axial load they receive, and the natural vibration frequency of their lateral vibrations decreases as the axial load increases. By measuring the time history waveform of the vibration response of seamless line steel rails under a certain transient excitation, it can determine their natural vibration frequency or vibration damping coefficient. Referring to the corresponding standard relationship curve, the actual longitudinal force value of the rail at the test point can be calculated; this predicts the stability of the seamless line track. The method of this invention can be used for multi-point practical measurements according to specific requirements, does not affect the track structure and train operation, can be tested while the rail is subjected to longitudinal tensile forces or compressive forces, is not affected by the locked rail temperature of the steel rail, is easy to implement, and is convenient to operate.