Abstract: By strict theoretical analysis and calculation, a mathematical model of synchronous motor of E-L equation in synchronous rotating coordinates was established and nonlinear model of inverter was given. Based on the models, a passivity asymptotical tracking controller of synchronous motor was designed in case of time-varying and unknown load torque(rotation rate and location). The controller realizes mechanical control target in meanwhile realizes control of magnetic-field direction and magnetic-field amplitude, namely realizes magnetic-field vector control. The simulation result showed that the passivity controller can realize tracking control of rotation rate and location, which has high dynamic control performance and robust.