In order to improve the computer simulation accuracy of vehicle dynamics and study the mechanical model of yaw damper and its effect on vehicle dynamics， a non-linear mechanical model of yaw damper of a high-speed electric multiple units （EMU） in China is established and analyzed based on the pressure-flow characteristics of compressible fluid. The results show that： compared with the traditional piecewise linear model， the nonlinear mechanical model of the yaw damper can simultaneously reflect the viscous damping force and the restoring force generated by the compression of the oil. Simulation calculation results are in a good agreement with the test results. Critical speed calculated by the nonlinear mechanical model of the yaw damper will increase first and then decrease as the equivalent taper of the tread increases. This will lead to a higher calculated lateral stability index and the trend of increasing with the speed increases is more significant. It shows that the nonlinear mechanical model of the yaw damper can effectively improve the accuracy of dynamic simulation， which has a greater impact on the stability of the snake's movement and lateral stability of the vehicle， but has less impact on the vertical stability and the safety of curve passing.