In GNSS-based InSAR systems, in order to obtain high azimuth resolution, more than 600s data need to be acquired, which results in large data amount. In this paper, a fast phase-preserving imaging algorithm based on segmented pulse compression is proposed to reduce calculation of GNSS-InSAR system. First, a segment echo signal processing model is established based on bistatic configuration. Then, calculation complex of pulse compression is reduced through data segments along range direction. Finally, pulse compression result for imaging is obtained by summing the segment result and image result is obtained by back projection algorithm. Raw data results suggest the effectiveness of the fast phase-preserving imaging algorithm.