A spatially extended gear crack model was developed, where crack extension varies simultaneously in the directions of both end face and tooth height. An analytical model of spatial crack was constructed based on the energy method. Using the constructed model, two different crack extension paths were simulated to investigate its influence on the time-varying meshing stiffness of gear. Furthermore, a dynamic model of the gearbox was established to analyze the effect of crack extension path on the dynamic performance of the gear system. The results showed that with the extension of spatial crack, the time-varying meshing stiffness (TVMS) decreased, while the vibration impact value of the gear teeth increased accordingly.