To solve the effect of the range imaging broadening, the speed estimation of high-speed non-cooperative target of inverse synthetic aperture lidar translated for the estimation of the frequency modulation slope. A parameter estimation method of fractional Fourier transform (FRFT) was presented. This method could effectively estimate the frequency modulation slope. Under low SNR, the error of the estimate was less than 0.03. The research laid the foundation for velocity compensation and 2-D range-Doppler imaging of the follow-up.