Abstract: ‍ ‍Spaceborne high-squint spotlight synthetic aperture radar （SAR） has wide application prospects because of its large detection range and long residence time. However， large range migration is produced， causing significant echo signal coupling. The traditional algorithm uses a fixed window to receive the data. For more complete data collection， a large sampling window is needed， but a lot of invalid data are also acquired， and the sampling efficiency is very low. To solve the problem of low sampling efficiency in the time domain， the sliding window receiving mode can be used. This mode is generally used in combination with the time domain algorithm or the Polar Format Algorithm. In the case of spaceborne SAR imaging in large scenes， the time domain algorithm has low computational efficiency and the Polar Format Algorithm has a small focusing radius. Therefore， this paper proposes an improved Range Migration Algorithm to address the problem of efficient and accurate imaging in high-squint sliding window receiving mode. Accordingly， the time-domain echo model in sliding window receiving mode is established， and the accurate two-dimensional analytic spectrum is derived via Fourier transform. Based on the two-dimensional analytic spectrum， a two-dimensional spatial domain decoupling method in the radar line-of-sight coordinate system is proposed and the corresponding complete imaging processing flow is obtained. Finally， the effectiveness of the algorithm is verified through simulation experiments. The proposed method can not only solve the problem of low sampling efficiency caused by the distortion of the two-dimensional spectrum of high squint， but also avoid the problem of residual distance migration amplification in the traditional method in the high squint mode， as well as provide convenience for the subsequent self-focusing processing.