Abstract: Squint sliding spotlight improves the multi-angle imaging capacity with the single pass and the imaging flexibility of spaceborne synthetic aperture radar (SAR). However, with the increase of the squint angle and the beam scanning angle in azimuth, it is difficult to solve Doppler aliasing caused by the additional bandwidth owing to the squint angle and the nonlinear instantaneous Doppler centroid. In this paper, a new spaceborne squint sliding spotlight imaging algorithm based on sub-aperture data preprocessing is proposed to solve this problem. The traditional sub-aperture data partitioning method is modified, and the remained Doppler aliasing in each block of sub-aperture data is resolved by two-dimensional spectrum mosaic and Doppler filtering. Afterwards, the refined chirp scaling (CS) algorithm based on the fourth-order slant distance model is adopted for SAR data focusing imaging. Finally, the proposed imaging algorithm is validated by simulation results.