An analysis method based on dynamic backlash for spur gear system with wear fault was proposed according to the characteristic of the time-varying backlash in gear system caused by tooth wear, eccentric and bearing vibration. Moreover, a meshing coupled dynamic model with 6 degree of freedom consisting of a single-stage spur gear system was developed involving time-dependent meshing stiffness, dynamic backlash, friction and eccentricity. And the simulation methods were given to analyze the dynamic behaviors of the gear system with uniform wear fault and eccentric wear fault. At last, the wear fault experiment with increasing backlash was performed on the gear experimental bench, which verified the theoretical analysis. Research results show that the gear transmission error, shock status and vibration intensity will change according to the form and degree of wear fault, which provides a theoretical basis for gear system monitoring and diagnosis with wear fault.