Abstract: According to the characteristics of airborne SAR motion compensation, the key factors that affect the motion compensation using INS(inertial navigation system) are analyzed and the PGA(phase gradient autofocusing) algorithm is discussed in detail and realized. In using both the INS data and autofocusing algorithm to compensate the motion error of airborne SAR, the former is used to perform the primary work, thus the high frequency component of the motion error can be compensated, while the latter is used to compensate the remaining low frequency error. PGA is chosen to do the autofocusing work. It can correct any order phase error accurately. In using PGA to autofocus the SAR image, the results showed its effectiveness.