The invention relates to a permanent magnet synchronous motor (PMSM) AC servo system, which is composed of a DSP control panel, a rotary transformer, a main power circuit and a conditioning and protecting circuit, a motor body consists of an armature winding and a permanent magnet core rotor, wherein the motor is excited by a permanent magnet, and the DSP is used for realizing an field-orientated control algorithm, communication, sampling calculation of current and configuration output of a drive; the output of the rotary transformer enables data to be transmitted to the DSP via four-path DA transform chips and SPI of the DSP or multi-channel buffering; the DSP outputs pulse width modulation, is connected with voltage signals output by a current regulator, and is connected with a PWM1-PWM3 drive inverter which can convert the voltage signals into three-phase switch signals by CLARK inverse transformation, and the inverter outputs to the armature winding so as to control the armature winding of the motor to generate a rotary round magnetic field; and the main power circuit is composed of a direct current detection circuit with controllable rectification, an inverter, a current detection circuit, an overvoltage discharging circuit, a protection circuit and a driving circuit. The invention adopts the field-orientated control algorithm which can control the torque, speed and position state of the motor in real time and has good control performance.