Spaceborne parasitic SAR is a typical form of distributed small satellite system.With theoretical derivations
it's found that improving range and azimuth resolution by combination of several images from different small satellites can be explained uniformly by scattering spectrum two-dimension shifting
and improving range and azimuth resolution can be completed at the same time.A processing method is given to combine multiple images for improving spatial resolution of spaceborne parasitic SAR
which compensates spectrum shifts in two directions and fixed phase difference.Regularity of cover area in frequency domain after combination is considered to assure to get high quality images in spatial domain.Simulated data show that theoretical derivations are in good agreement with practice.