In order to calculate gear reliability， the failure probability models is established aiming at the two common failure modes of wear failure and fatigue failure， respectively. The reliability is calculated based on the results of the failure probability. The effects of the shocks following the Poisson process on wear and contact stress are considered. The wear-wear threshold interference model is established in the form of cumulative wear. Considering random strength degradation， the dynamic strength-stress interference model is established. With a numerical example for gears in the reducer， the reliability analysis is conducted. Verified by this numerical example， the gear reliability results of the two failure modes considering the shock are obtained. Additionally， reliability values under different shock parameters are analyzed. The results show that the model can provide specific guidance to life estimation， material selection and other engineering issues.