Abstract: Motion error problem for airborne formation flying InSAR is analyzed. The plane interval changes (across track baseline changes) are discussed in three situations: interval increasing, interval decreasing, interval changing according to sine curve. Phase changes with baseline changes are derived quantificationally. The simulation result of plane interval changes and velocity difference are given out. Azimuth slope eliminating is adopted. Two control points are set in topographic area to compensate the phase error of baseline changes. This algorithm can improve the digital elevation model precision for whole area. The simulation results prove that the analysis is right.