Abstract: Taking the involute spur gears as the object, a single degree of freedom gear torsion model and a multi-degree of freedom torsional vibration model considering transmission shaft torsional stiffness were established based on the time-varying meshing stiffness and error excitation factors. The equivalent comparison formula between transmission shaft torsional stiffness and gear meshing stiffness was derived. The system dynamic differential equations of the two models were solved through software, and the dynamic response of the two types of gear system were obtained which mainly including the variation curves of dynamic indicators such as relative meshing displacement, relative meshing velocity, tooth surface dynamic normal load, dynamic load coefficient, etc. By comparing and analyzing the calculation results, it is shown that the step mutation of time-varying meshing stiffness will bring dynamic impact to gear transmission; the error excitation will exacerbate the dynamic response fluctuations of the system and increase the dynamic load coefficient of the system; series connection of torsional stiffness and meshing increases the flexibility of the system and intensifies the dynamic fluctuations of the system response. The coupling of error excitation and torsional vibration of the transmission shaft increases the vibration amplitude and oscillation frequency of the system, and the dynamic load coefficient significantly increases, which has a significant impact on the stability of the system. The research results provide fundamental support for further in-depth research on the dynamics of gear systems.