A novel method to control the pulsed light extinction ratio automatically, which combines a double electrode series structure EOM with “scanning & step-tracking” algorithm, is proposed. It aims to address the problem that extinction ratio of pulsed light is affected by bias voltage shifting in a Brillouin Optical-fiber Time-domain Reflectometry (BOTDR) based on distributed optical fiber sensing system. Based on the method, some experiments are conducted and important conclusions are deduced. The results show that it could keep the extinction ratio of pulsed light above 50 dB steadily. The fluctuation of extinction ratio is only 1.3 dB, maximum value of extinction ratio is 52.4 dB and the minimum value is 51.1 dB. It is indicated that the proposed method will have important significance for enhancing the performance of pulsed light source in BOTDR optical fiber sensing system and high power laser driving device.