The squinted sliding spotlight synthetic aperture radar (SAR) based on the fixed pulse repetition frequency (PRF) and continuously varying PRF will lead to the problems of blind ranges and azimuth non-uniform sampling. Fortunately, the block varying PRF can avoid the above two problems in the whole acquisition interval simultaneously, but it will cause the azimuth sampling intervals of the data blocks corresponding to different PRFs to be different. In addition, the squint angle will introduce a new Doppler spectrum aliasing phenomenon. This makes the conventional two-step preprocessing algorithm infeasible. To resolve these problems, a preprocessing approach of squinted sliding spotlight mode based on block varying PRF is proposed. In this method, data blocks corresponding to different PRFs are first subjected to the range frequency-dependent de-twist and de-ramping to resolve Doppler spectrum aliasing caused by the squint angle and azimuth beam steering. Afterward, different numbers of azimuth zero-padding are introduced into the obtained raw data to make the azimuth sampling intervals of the output data blocks corresponding to different PRFs consistent. Furthermore, the signal after azimuth combination should be resampled to make azimuth sampling interval at different range frequency equal. Finally, the feasibility of proposed preprocessing method is verified by simulation experiments on point targets.