The phase errors due to the nonlinear chirp of a tunable laser reduce the range resolution in synthetic aperture imaging ladar (SAIL). The compensating algorithm was developed. The estimation errors of the phase shifting function and the compensation errors of the traditional compensation algorithm must be eliminated. The theoretical derivation of the practical phase shifting function for finite data in practical application is established and the reason for the transfer errors of the theoretical phase shifting function is analyzed. The simulation model of the airborne SAIL working in strip map mode is established,and the simulation validates the derivation results. The method for reducing errors in the nonlinear chirp compensation algorithm is given finally.