During the measurement of a BOTDR system, it is significant to keep the extinction ratio of probe pulse uniform if constant sensing performance is desired. However the bias point of an electro-optic modulator (EOM) may slowly changes due to many external factors. Thus the variable bias point of the EOM must be stabilized. In this paper, a novel and simplified stabilization technique based on digital signal processor and step-by-step tracking algorithm is proposed. Contrast experiments with and without the stabilization are performed. The experimental results show that our method can lock the EOM’s bias point quite well and improve the sensing system’s performance obviously.