The nonlinear chirp of laser reduces the range resolution of synthetic aperture imaging ladar. Based on the nonlinear chirp measurement in reference channel and phase-shift algorithm, a compensation method is proposed, which filters and proofreads the target backwave one by one, and mathematical flow of this method is established. The simulation of the airborne synthetic aperture imaging ladar model validates the feasibility. The effects of width of the scan filter and nonlinear chirp are discussed. The conclusion of adequate width of the scan filter is given.