The technique of drift-scanning is efficient in working, but the observation of drift-scanning mode would defocus the imaging energy inevitably. The models for analyzing the value of the defocus quantificationally was built, which provides the theoretical reference for the evaluation and improvement on the image quality of spatial targets. Firstly, the principles of both drift-scanning and spatial projection imaging were analyzed. Then, the relevant spread models of targets’ energy were set up, which were used to give a quantificational analysis individually on the defocus of non-synchronization caused by the charge’s mismatch with star image in the moving velocity, and the defocus of track-projection brought by the spatial targets’ curve movement. Finally, a simulated experiment was made to validate the feasibility of the relevant defocus models, whose applicability were also discussed.