Phase-sensitive optical time domain reflectometry (-OTDR) has the advantages of fast response and high sensitivity. Therefore, it can realize fully distributed monitoring of weak vibrations along an optical fiber, which is of great value in many applications such as perimeter security and structural health monitoring. However, the optical background noise in the -OTDR will disturb the extraction of effective signals and limit the performance of this system. The optical background noise mainly includes the laser center frequency drift, the polarization-relevance noise and the distortion measurement due to the nonlinear relationship between optical fiber strain and interference intensity. In this paper, the generating mechanism of these optical background noise was analyzed and the corresponding noise suppression methods were proposed. The experiment results showed that the proposed methods could suppress the optical background noise effectively and improve the sensing performance significantly.