We propose a novel method to control the electro-optic modulator (EOM) applied in the phase-sensitive optical time-domain reflectometer (ϕ-OTDR) distributed sensor system, which uses the data of the OTDR curves rather than applying an independent control module. We explain the relationship between the accumulation value of the OTDR curve and the EOM's extinction ratio, and utilize this relationship to feedback control the EOM. The experimental results show that it can compensate the drift of the EOM, and make the modulator run with a high extinction ratio for a long time. And this method can also ratify a small jump of the EOM's bias point.