The invention discloses a system device and method for controlling the bias voltage of an electro-optical modulator, provides an automatic bias control system based on a Sagnac ring and a direct-current power detecting algorithm and mainly relates to the field of the optical communication technology and automatic control. According to the system device and method for controlling the bias voltage of the electro-optical modulator, an optical carrier is split into two paths at the output end of a light source with a polarization controller and a polarization beam splitter and enters the Sagnac ring, wherein the clockwise optical carrier is subjected to intensity modulation, while the anticlockwise optical carrier is not modulated with radio-frequency signals due to the reverse modulation characteristic of the electro-optical modulator; optical signals output by the Sagnac ring pass through the polarization controller and the polarization beam splitter so as to make the unmodulated optical signals enter a bias control module through a photoelectric detector, judgment and compensation can be carried out on direct-current drifting with a direct-current power detection method, and thus a direct-current working point of the modulator is locked. According to the scheme, the defect that the direct-current power detection method is affected by radiofrequency signals is overcome with the Sagnac ring, and automatic bias control irrelevant to the radiofrequency signals is achieved.