Beat-FFT is usually used in the Millimeter Wave(MMW) Linear Frequency Modulated Continuous Wave(LFMCW) system. The beat signal contains velocity and range information when there is relative motion between target and radar. The effect of decoupling by FFT transform is not ideal. In order to solve this problem, Fractional Fourier Transform(FRFT) is introduced as well as the principle of LFM signal detection based on FRFT. An improved local FRFT spectrum refinement method is put forward. Simulation results show that the frequency spectrum refinement algorithm can obtain higher spectral resolution under the same points of FFT transform; and it would take less calculation to get the same spectral resolution.