Abstract: After the clutter suppression, the space target echo of the space-based early warning radar still has inter-pulse envelope migration and Doppler diffusion, which causes the echo energy to defocus in the distance unit and Doppler domain after coherent accumulation. In order to increase the gain of coherent accumulation, the relative motion parameters between the satellite platform and the air target must be estimated to compensate the echo signal. Due to the limited power aperture product of the spaceborne radar and the long distance between the radar and the target, the signal-to-noise ratio of the echo signal is generally low, which affects the estimation accuracy of the motion parameters. Aiming at this problem, this paper proposes a method for estimating the motion parameters under low SNR. Firstly, the echo model of the air target is established, and the echo signal-to-noise ratio range in the radar line-of-sight range is calculated. Then, the Keystone transform is used to correct the distance migration of the echo envelope to obtain the azimuth timedomain signal; finally, the timefrequency distribution of the azimuth time-domain signal is obtained by Gabor transform, and then the time-frequency domain is transformed from the time-frequency domain to the parameter domain by standard Hough transform, and finally the estimated value of the motion parameter is obtained. The simulation results show that the proposed method can effectively estimate the target motion parameters under low SNR, and the estimation accuracy is not sensitive to the change of SNR, and it has good robustness.