Due to wind turbulence and flight instability, the flight trajectories of drones often exhibit highly variety in position and attitude. The deviation from linear stable paths causes non-uniform illuminating energy of Synthetic Aperture Radar (SAR) system on targets, and also the fluctuant SAR image amplitude. In order to suppress the effect of imaging model and revert the scattering features of targets, in this paper the concept of illumination is introduced to denote the radar illumination energy per unit area. Then the expression of illumination is deduced, and shows that image amplitude can be considered as the product of illumination and scattering intensity of targets. Traditional image equalization methods confuse the difference between illumination ununiformity and amplitude of targets, thus the equalization results can not show the real scattering intensity of targets. A novel image equalization method based on illumination is presented to overcome these conditions unconsidered by previous algorithms. In this method the illumination is computed during the Back-Projection (BP) accumulation and the image result is weighted by the illumination. The computer simulation and real data results demonstrate the effectiveness of this approach in suppressing the image amplitude ununiformity caused by variety in position and attitude and resuming the scattering feature of targets.