The non-ideal characteristics of central electronic equipment and phased array antenna in Synthetic Aperture Radar(SAR) system can introduce systematic errors, which may contribute to amplitude and phase errors of SAR signal, and hence degrades SAR images. A model is presented to derive signal errors due to non-ideal characteristics of central electronic equipment and phased array antenna, and a method is proposed to correct the errors. The model shows that the error resulting from central electronic equipment is almost invariant, while the error resulting from phased array antenna varies with the direction of the beam and is modulated by the attenuation and phase shift amount of T/R. The system error of phased array antenna SAR can be extracted by system testing or analysis calculation, and then can be compensated during SAR imaging process.