Abstract: To calculate the time-varying mesh-stiffness of fault gear effectively and accurately,the improved energy method based on the traditional energy method was proposed.The method combined advantages of both the improved energy method and the finite element method i.e.conducting fast calculation of meshing stiffness of normal teeth of fault gear by improved energy method while computing meshing stiffness of fault teeth by finite element method.Simulation results prove that comparing with traditional energy method and finite element method,the method can calculate the time-varying mesh-stiffness of fault gear fast and accurately.