The invention relates to a modulation ratio control method based on voltage closed loop of a flywheel energy storage asynchronous generator. The modulation ratio control method comprises the steps of comparing a direct current bus voltage given value with an actually detected feedback value, and obtaining a modulation ratio given value after the comparison result passes through a PI controller and an amplitude limiting module; meanwhile, giving the stator frequency; calculating and obtaining a carrier wave ratio through a flywheel speed reduction curve and an f/fc unit (as shown in the specification); then calculating the turn-on time of a switch tube according to the modulation ratio and the carrier wave ratio to generate an SPWM wave for controlling on and off of a power converter power module so as to further control the frequency of a high-speed asynchronous motor stator dragged by the flywheel to enable the frequency to be lower than the rotating speed of the flywheel constantly; and enabling the motor to be an energy feedback state so as to convert the mechanical energy stored by the flywheel into electric energy to realize power generation. According to the method, the modulation ratio is taken as the main control quantity; the stable output of the power generation voltage can be realized, and the rotor position information is not required to be detected; and meanwhile, the control algorithm is simple and easy to implement.