Phase-sensitive optical time domain reflectometer (-OTDR) has been a research hotspot in recent years. For -OTDR, the Rayleigh backscattering light is monitored with a photodetector, and the plotted trace is a curve of the returned optical power versus time. The time is proportional to the distance from the location along the sensing fiber to the optical pulses’ incident end. The trace presents the form of speckle-like profile owing to the coherent addition of the amplitudes of the light backscattered from different locations of the fiber. Based on this, the influences of the intrusion signal’s characteristics (occurrence location and phase variation) on the detection performances of the distributed sensor based on -OTDR are investigated theoretically and demonstrated experimentally, especially on signal-to-noise ratio (SNR), missing report, and location accuracy. Results show that if intrusions occur at locations corresponding to the curve’s peak or subpeak and when its phase variation is , SNR will be the maximum. If intrusions occur at locations corresponding to the curve’s troughs, a missing report will occur no matter what value is taken by the phase variation of the intrusion signal. On the other hand, experimental results show most missing reports can be uncovered by using a frequency tunable laser and/or optical pulses with an alternative repetition frequency.