A distributed fiber optic intrusion sensor system based on Brillouin optical time domain analysis (BOTDA) is proposed. The absolute values of temperature and strain are not required in this system. So the complicated frequency analyzer is omitted,which simplifies the BOTDA system. An optical pulsed signal is used as pump and a continuous wave optical signal in counter propagating direction is used as the detected light. It is concluded that the best sensitivity can be achieved when the frequency of continuous wave is higher than the Brillouin peak gain frequency produced by pump and there exists an optimal frequency offset that makes the highest sensitivity. When the temperature of the optical fiber changes,the optimal sensitivity can be achieved by adjusting the pump frequency.