In order to keep the input light polarization direction of electro-optic modulator(EOM) up with that of internal propagation mode in an EOM, and to obtain expected modulation effect, an adaptive control method based on proportional-integral-derivative(PID) algorithm was proposed. Theoretical analysis and experimental demonstration was conducted to verify the feasibility of the proposed method. The experimental results show that the control system designed by the method not only can real-time monitor the output optical power of EOM, but also can keep the output power maximum. Compared with the performance of EOM conncted directly to a narrow band laser with a polarization maintaining fiber, the output average power of EOM under the polarization control based on PID is increased by 3.08 dB and the standard deviation of PD output voltage is reduced from 0.121 4 to 1.237 5×10^-4. And the correction time for the state of polarization is in the order of ms when the system is working in stable control state.