The invention discloses a bias voltage control device of an electrooptic modulator and a control method thereof. The bias voltage control device is a bias voltage drive developed aiming at the electrooptic modulator. By controlling bias voltage of the electrooptic modulator, the aim of ensuring that the electrooptic modulator stably works at a proper work place can be achieved. A photoelectric detector (1), a transimpedance amplifying circuit (2), an analogue-digital converting circuit (3), a controller (4) and an output drive circuit (5) of the bias voltage control device are connected in sequence; pulse light modulated by the electrooptic modulator splits out a part of light signals through an optical coupler to be connected with the input end of the photoelectric detector (1); and the output end of the output drive circuit (5) is connected with a bias voltage input port of the electrooptic modulator for controlling the bias voltage of the electrooptic modulator. The device realizes the automatic control of the bias voltage of the electrooptic modulator, so that the electrooptic modulator continuously works at stable extinction ratio and the work stability of the electrooptic modulator can be improved.