By investigating distribution regularity of measuring error theoretically and experimentally, this paper builds measuring error mode of phase modulation laser Doppler shift measuring method based on its operating principle, which can be used to quantitative analyze and optimization design of Doppler shift measuring method for acquiring optimum measuring results in practical application. This error mode is utilized to theoretically investigate the influence of phase modulation depth and frequency on its frequency shift measuring accuracy and so on. It is found that measuring accuracy can reach the maximum when phase modulation frequency is equal to 0.63 times of full width of half maximum of Fabry-Perot interferometer transmission curve and phase modulation depth is 1.08.