Abstract: Fiber -optic distributed disturbance sensor based on Phase -sensitive Optical Time -Domain Reflectometry (-OTDR) can achieve real -time intrusion detection on a large scale. As a critical component of the sensor, the parameters of the laser affect the monitoring length and the spatial resolution. In this paper, the theoretical analysis and the experiments about the influences of the laser source on the system was described. The investigation results indicate that the monitoring length increases with the rising of optical power of the laser, the pulse width and the modulation period; the spatial resolution decreases with the decline of pulse width. Based on the above analysis, the preferred selection of the laser parameters with different monitoring length is proposed. The theoretical analysis are proved by the experiments.