Mesh stiffness is the most important internal excitation source in spur gear transmission system， and the acceleration response characteristic caused by it is an important basis for state monitoring and fault diagnosis for gear system. Firstly， the full finite element dynamic model of gear system is established with 2-node beam element， which is used to overcome the deficiency of the one using lumped parameter method traditional. Secondly， considering that the base circle and the root circle of the gear do not coincide usually， the model of the complete tooth profile curve is obtained based on the general tooth profile curve equation， and then the curves of mesh stiffness under different failure modes are obtained. At last， the numerical method is used to solve the dynamic model， and the results are compared with the experimental observation. The simulation results show that， under the condition of the local gear fault， impact component spaced during gear rotation period appears in the time-domain， and sideband spaced during gear rotation frequency appears in the frequency domain， and the strength in both of the two domains is positively related to the degree of gear local fault. The acceleration signal obtained in experiment is consistent with the theoretical simulation results， which proves the correctness of the method.