In order to reveal the influence of rotational speed changes on the dynamic characteristics of High Contact Ratio (HCR) spur gear, the calculation model of HCR gear’s time-varying meshing stiffness based on potential energy method was established, and the variation rules of HCR gear’s time-varying meshing stiffness was obtained. The system dynamics model including shaft segment, HCR spur gear pair, bearing, etc., was established, the time-frequency characteristics of dynamic response of HCR gear system and ordinary gear system under different speed conditions are compared and analyzed. The results show that compared with the ordinary gear system, the RMS and peak-to-peak values of dynamic meshing force and vibration acceleration of HCR gear system decrease significantly under different speed conditions, and the vibration acceleration of each measuring point increases radially with the increase of input speed.