The stiffness of the intermediate shaft connected between the two stages affects the working condition of the planetary gear drive system. The secondorder spur gear system of a test bed was used as the research object, and the secondorder gear shifttorsion dynamic model was built, the influence of the change of the stiffness of the intermediate shaft on the dynamic characteristics of the system was discussed. The results show that the change of the stiffness of the intermediate shaft is more obvious for the dynamic load of the adjacent two teeth connected by the intermediate shaft. With the increase of the connecting stiffness of the intermediate shaft, the dynamic load of the tooth is gradually decreased. At the same time, the influence on each frequency component and its frequency doubling is obvious, and the energy of each frequency component and its frequency multiplication component decreases linearly with the increase of connection stiffness. The change of the stiffness of the intermediate shaft connection causes the natural frequency of the system and its mode to produce mode transition.