Most of control algorithms for permanent magnet synchronous motor(PMSM) are based on mathematic model of the motor, which quality impacts on control performance directly. In order to deal with model mismatch for servo PMSM, the mechanism of motor was analyzed from both kinematics and dynamics, and corresponding mechanism model was obtained. Combined with experimental data, based on given performance index, related parameters were computed online by using an iteration algorithm to obtain corresponding mathematic model. The convergence of iteration algorithm was analyzed finally. Simulations and experiments demonstrate that the proposed modeling method is simple and easy to implement, and can obtain quite accurate model of the motor fastonline.