Phase-Derived-Range, a radar range method based on echo phases, is highly evaluated for its root-mean-square (rms) error within half of wavelength and great used on highly precision ranging for micro-motion target. There are two technical challenges, which are ambiguous Doppler phase acquisition and ambiguity resolution. A method based on envelope migration is proposed in this paper. Firstly, ambiguous phase was extracted from the 1-D range profile; secondly, the ambiguity was resolved by the envelope migration; then, the velocity was estimated by least squares method and the high-precision range was acquired by compensating the error term. In this method, the phase error caused by velocity is compensated at last instead of the ambiguous phase, so that the velocity error does not affect the correctness of ambiguity resolution. Therefore, this method enhances the applicability of this algorithm. The limitations have been provided. Simulation and experiment results validate the performance of the method.