The reasons of the generation of defocus were discussed and the defocus change was analysed from the relative distance, atmospheric pressure and temperature change. The modulation transfer function model was proposed to study the image quality influenced by defocus. Then different defocus value leading image blurry was simulated. The simulation results indicate that the positive and negative defocus value induced by the relative distance, atmospheric pressure and temperature change can be counteracted. This paper provides an effective data to spatial camera design, compensating measure establishment and performance evaluation.