A novel automatic bias control algorithm based on in-phase quadrature-phase silicon-based optical modulator is theoretically and experimentally proposed. Firstly, we compare the modulation characteristics between silicon-based optical modulator and lithium niobate modulator. Based on the model of in-phase quadrature-phase silicon-based optical modulator, we derive the functional relationship between the output signal and the electric field signal of the modulator. A control scheme is proposed to add the sinusoidal dither signal into the bias operating point of the modulator and analyze feedback signal spectra of the low-speed monitoring photoelectric diode which is built-in modulator. The feasibility of the scheme is verified through numerical simulation. Then we build the experiment platform of automatic bias control. We obtain the test results of monitoring photoelectric diode signal spectra with different bias voltages applied to channel I (in-phase), channel Q (quadrature-phase), and channel Phase (outer-phase) by using the experiment platform. The model of the photoelectric transfer function described in this paper is supported by simulation and measurement results. Finally, we measure the control accuracy of the algorithm in 128 Gb/s dual-polarization quadrature phase shift keying modulation format and calculate the penalty factor of the optical signal-to-noise ratio by the vector magnitude error parameter.