Abstract: Since factors such as turbulence and pilot error make it impossible for an aircraft to flight along the nominal straight flight path, SAR motion compensation technology is crucial to acquiring high quality image. When the aircraft changes in the roll, pitch or heading, it is necessary to track the real time attitudes to stabilize the antenna pointing. While the strapdown inertial navigation system can fulfill the requirement. The instant attitude updating is one of the main tasks of attitude measurement for strapdown system. Consideration of the real time ability and stable performance, a new derivation method of the transformation equations between quaternion and attitudes is proposed. The influence of angle motion in airplane on SAR imaging is analyzed based on the derived equations. The method given in the paper can highly reduce the operation of triangle function and improve the efficiency of the system.