The airborne multiinput multioutput synthetic aperture radar (MIMOSAR) may have spacevariant range error while enlarging the illuminated areathus result in various imaging error for the existing algorithms applied to the large scale model.This paper presents a theoretical analysis based on the sidelooking modeland the analysis shows that under the widen swaththe mismatch between the echo signal and the reference function is the main factor of the imaging error.So an algorithm of highprecisionlarge region imaging was proposedwhich was verified by the results of simulation.