Laser phase modulation spectrum with 25 frequency bands is generated by modulating a single frequency laser with two cascaded phase modulators (PMs) with driving voltage amplitudes at 3.2 V and 7.8 V, respectively. And the time delay self-heterodyne method is adopted to measure and analyze the coherent characteristics of the original single frequency laser light and the generated multi-frequency light from two phase modulation schemes. By comparison of laser linewidth, the experimental results show that the laser phase modulation does not change the coherent characteristics of each frequency band, and the laser phase modulation spectra benefit the performance optimization for the Rayleigh scattering based optical fiber sensing system.