The error of needle tooth radius and the error of needle tooth center circle radius are important factors affecting the lost motion of RV reducer, and the error combination also affects the lost motion of RV reducer. To study the influence law of different error combinations， the influencing factors of lost motion were analyzed, and the calculation model of lost motion was established. Taking RV-80E as an example, the 3D model and dynamic model of RV reducer were established, and the lost motion simulation analyses under different error combinations were carried out. The simulation analysis and theoretical calculation results show that the theoretical calculation curves of lost motion are consistent with the simulation curves, which decreases with the increase of needle tooth diameter and increases with the increase of needle tooth center circle diameter.