To address the problems of unclear internal load evolution law of the electromechanical composite transmission system for high-speed tracked vehicles and the lack of fatigue life prediction method for planetary gear bearings,a dynamic model of planetary gear bearings in the electromechanical composite transmission system was established to obtain the distribution law of contact loads on planetary gear bearings. A fatigue life prediction method based on the dynamic load characteristics of planetary gear bearings was proposed, which provides certain guiding significance for the optimization and design of the planetary mechanism in the electromechanical composite transmission system. Considering the effects of multi-row coupling effect of the electromechanical composite transmission system, gear time-varying meshing stiffness excitation,and nonlinear support stiffness excitation of planetary gear bearings, a dynamic model of the electromechanical composite transmission system was established using Simpack to obtain the load-bearing conditions of planetary gear bearings.Furthermore, a dynamic model of planetary gear bearings was established using the lumped mass method to analyze the evolution law of contact loads on planetary gear bearings. Then, a fatigue life analysis and prediction model for planetary gear bearings was established using the L-P formula to analyze the variation law of fatigue life of cylindrical roller bearings for planetary gears under different working conditions. The results show that at high rotational speeds, the centrifugal force of the planetary gear bearings during revolution has a significant impact on the contact loads and fatigue life of the planetary gear bearings. At lower rotational speeds, the service life of the planetary gear bearings in the reduction gear increases with the rotational speed. At higher rotational speeds, the service life of the planetary gear bearings in the reduction gear decreases with the increase in rotational speed. At high rotational speeds, measures such as profile modification of the rolling elements of planetary gear bearings can be taken to extend the service life according to the life requirements of the electromechanical composite transmission system.