The influences of frequency drift of laser source on phase sensitivity optical time domain reflectometer(φ-OTDR) was investigated. One-dimensional pulse-response model of Rayleigh backscattering in a single-mode fiber was employed to analyze the trace-to-trace fluctuations induced by frequency drift. In laboratory test, an unbalanced Mach-Zehnder interferometer(MZI) with path-length difference of 100 m was employed to monitor the frequency drift real-time, and the performances of φ-OTDR system using three lasers with different frequency drift rate were compared to testify the theoretical analysis. Both the theoretical and the experimental results show that the frequency drift of laser source is an important source for the fluctuations of φ-OTDR waveform, and the trace-to trace fluctuations increase as the growing of frequency drift. Moreover, it is difficult to distinguish the fluctuations induced by frequency drift and the fluctuations induced by intrusion event in time-domain when the frequency drift is up to several hundreds of MHz/min. However, the differences between them are more evident in frequency-domain. The conclusion is useful for choosing laser sources and improving the performance of φ-OTDR.