The invention discloses a transmission permanent magnet alternating-current generator system and a voltage stability control method thereof. The system comprises a permanent magnet alternating-current generator, a PWM rectifier, a direct-current filtering device and a controller. The controller comprises a rotor position detection module, a direct voltage detection module, an alternating current detection module, a coordinate conversion module, a voltage loop regulator, a current amplitude limiting module, a quadrature axis single-current-loop regulator and a PWM modulation module. The method comprises the steps of collecting the rotor position and rotating speed of a generator, direct voltage and alternating current, obtaining quadrature-axis current through coordinate conversion, generating quadrature-direct axis voltage set value and performing direct-axis current amplitude limiting through closed-loop control aiming at stabilizing the direct voltage, and generating PWM trigger pulse for controlling the PWM rectifier. The output direct voltage is kept stable when the generator is operated at high rotating speed and the rotating speed changes, dynamic response is rapid, and the method is simple and high in reliability.