The change rule of phase-modulation beat-frequency-signal following signal light frequency was analyzed theoretically. It is found that the frequency spectrum of phase-modulation beat-frequency-signal phase is composed of two monotone edges separated from each other in domain. The results indicate that the beat-frequency-signal phase can be used to measure Doppler shift, and its Doppler shift measurement range is two times of Fabry-Perot interferometer edge technique if outgoing laser work-point is locked using beat-frequency-signal amplitude. Theoretical analysis of beat-frequency-signal amplitude and phase extraction methods shows that the measurement method based on beat-frequency-signal phase does not need signal light energy detection, and has simpler structure and less one outside noise entering channel than Fabry-Perot interferometer edge technique. For quantitative research and parameters optimization of this Doppler shift measurement method, its measurement error formula was deduced by using the error combination principle and computer simulation. The experimental system is set up based on the measurement principle. The output light of frequency tunable fiber laser simulates Doppler frequency shift signal light to be measured. The measurement results of frequency-discrimination parameter and its error distribution curves are agreed with the theoretical results well.