With its extending applications,Synthetic Aperture Radar(SAR) is required to process more and more data. The processing speed of traditional SAR processor turns out to be a bottleneck of the extension of SAR applications. Highly efficient SAR signal processors need to be explored towards these challenges. Recently,Graphics Processing Unit(GPU) is developing at top speed,which is a novel and promising computation platform for efficient SAR processors. A GPU-based SAR signal processor is introduced in this paper. The implementation of this processor takes full advantage of GPU′s computation capability, using general purpose computations to implement the SAR imaging based on CUDA programming model. As the result shows, this processor is more than ten times as fast as traditional CPU-based SAR processor. It provides a promising way to solve the problems of SAR signal processing in the future.