One of the main defects that influences the lifetime of a gear are cracks. The phenomenon induced by crack initiation and propagation is the continuous degradation of meshing stiffness, which will determine the response feature of the entire gear transmission system. Based on the assumption that the gear tooth is simplified as a variable cross-section cantilever beam, a time-varying meshing stiffness calculation model is established, in which the tooth bending, basis deformation and contact deformation are all included. The meshing stiffness curve of the healthy gear is obtained with comparative analysis between the above analytical model and FEM simulation. Furthermore, on the basis of a tooth root crack propagation prediction from the linear elastic fracture mechanics theory, the FEM models of gear tooth with cracks of different lengths are found. The stiffness calculation within one and a half meshing periods for each model is completed, and the relationship between crack size and stiffness degradation trend is established. The comparison between degradation trends in different meshing regions shows that the degree of degradation in a single tooth meshing area is much higher than that in a double teeth meshing region. In addition, there is some difference between the double teeth meshing areas.