Abstract: Inverse scaled Fourier transformation (ISFT) algorithm uses the inverse scaling property of ISFT to perform the range cell migration correction (RCMC) of SAR echoes in two-dimensional frequency domain in order to improve the corrected precision of RCMC and the computational efficiency, so it has become an important frequency domain algorithm. The paper studied the processing procedure of the algorithm, and redefined the expression of the chirp rate used to the range compression, which made the algorithm to accommodate secondary range compression (SRC). Further, one-order term coefficient of the range frequency in ISFT algorithm was modified. The modified ISFT algorithm is suited for the demands on data processing of squint mode SAR. The simulation tests validated the modified ISFT algorithm to process the squint mode SAR data.