Abstract: Phase-derived velocity measurement (PDVM) can obtain phase level measurement accuracy, but there are two difficulties in extracting the echo phase and resolving the phase ambiguity. In this paper, a phase-derived velocity measurement method is proposed for the variable pulse repetition interval waveform under low signal-to-noise ratio (SNR). Firstly, an echo model is established with considering the factor of pulse repetition interval variable. Then, the PDVM model based on wideband envelope velocity measurement (WEVM) to solve the phase ambiguity is introduced. In order to improve the applicability of PDVM method under the condition of low SNR, the extracted echo phase value correction method and the phase ambiguity number correction method are put forward respectively. The extracted echo phase value correction method utilizes the track filtering process to smooth the velocity measurement results, which enable to solve the problem of inaccurate extracted peak phase under low SNR. The phase ambiguity number correction method iterates through the phase ambiguity number and then minimizes the corresponding velocity fluctuation measurement error. Finally, the validity of the proposed method and its applicability under low SNR are verified by simulation.