By steering antenna beam in along-track direction,space-borne synthetic aperture radar (SAR) can achieve wide swath coverage in Terrain Observation by Progressive Scans (TOPS) mode.In TOPS mode,the beam switches in range and steers along azimuth several times to perform wider swath both in range and azimuth.In order to obtain a whole imaging result with uniform azimuth intervals,the re-sampling operation is usually required,which means large computation load and low processing efficiency.In this paper,a modified three-step algorithm based on Graphics Processing Units (GPU) is proposed.Firstly,a new interpolation method is introduced into the de-ramp operation (the third step of the three-step focusing algorithm),which avoids the up-sampling operation and can conveniently adjust the azimuth interval of final images.Then,a parallel framework of GPU is designed to accelerate the imaging processing exponentially.Finally,experiments based on simulation data are conducted to verify the validness and efficiency of the imaging algorithm.