Abstract: The physical meaning of each Doppler frequency shift items in azimuth direction caused by target motion is explained firstly. The analytical relationship between the peak position of the moving target’s strip image for airborne side-ooking mode SAR in azimuth direction and the initial position, velocity and acceleration are obtained using stationary phase principle. The theoretical expressions of the offsets and spreading cross range of each phase modulation items, such as quadratic phase factor, cubic phase factor, and quartic phase factor, are derived. Through separation of the phase modulation items and motion parameters, the theoretical derivations are verified by the simulation experiments. The research results are useful to understand the change of moving target’s SAR images.