In a Brillouin optical-fiber time-domain reflectometry(BOTDR) system,it is convenient to obtain the Brillouin signal by coherent detection,which needs an electro-optic modulator(EOM) to shift the frequency of the reference signal.However the bias point of the(EOM) may slowly change due to many external factors,which leads to the instability of the modulation efficiency.In this paper,a novel and simplified stabilization technique based on digital signal processor and step-by-step tracking algorithm is proposed.Experimental results show that the EOM′s bias point can be locked quite well in 24 h and the accuracy and the stabilization of the sensing system can be improved obviously.