Velocity measurement is a basic task of radars. It is prerequisite for tracing and imaging. In this paper, a novel method is proposed to measure target velocity with high accuracy for linear frequency modulation (LFM) radars. Firstly, the new method implements the stretch processing of received signals to obtain intermediate frequency (IF) signals. Then the difference processing is applied to IF signals in both the fast time domain and slow time domain to get the second differential phase. Finally, the second differential phase is utilized to estimate the velocity. Theoretical analyses and simulation results verify that the proposed method achieves better estimation accuracy and is robust in a wide range of bandwidth than the traditional Doppler method.