The shadow edges in SAR images are blurred. The error analysis for shadow edges defocusing is the theoretical basis of the shadow refocusing and target shadow information extraction. Taking the range Doppler algorithm for example, shadow quadratic phase errors of secondary range compression and azimuth compression are deduced in this paper according to the formation mechanism of shadow, which establish the relationship between the defocusing error and imaging parameters. The azimuth compression is the main step to affect the defocusing error, while secondary range compression is another. The defocusing error is related to the radar frequency, the radar resolution and the height of the target. For the Xband and Kuband radar with a resolution worse than 0.07 m, the focusing error of the shadow edges caused by the azimuth compression would reach 99.0% of the total errors, where the refocusing in azimuth is required. When the resolution of the Kuband radar is better than 0.04 m, the optimization of azimuth compression and secondary range compression are both required.