Large errors in time-varying mesh stiffness calculations of steel-plastic gears can result from the use of the potential energy method.Therefore, this paper considers the meshing characteristics and material properties of the steel-plastic gear pair, and proposes a method to calculate the time-varying mesh stiffness. The influence of load change on the time-varying mesh stiffness and dynamic performance of the steel-plastic gear pair is also considered. The simulation results show that the stiffness changes linearly with the load. The dynamic performance of the steel-plastic gear pair deteriorates with the increase in load. Under the same load condition, the dynamic response considering the effects of premature and extended tooth contact is better than that of the gear pair that does not consider the premature and extended tooth contact; this advantage is more evident at higher load.