In this paper,the focus measurement of a linear CCD space remote sensing camera in push-broom mode is studied by using a digital image processing method.Because there is no overlap between two images obtained continuously by the linear CCD space remote sensing camera in push-broom mode,it is difficult to select the evaluation function of image definition.According to the optical transfer function theory,the rough focus adjustment evaluation function based on blur estimation is established. From two images at different defocus positions,the defocus can be estimated and hence the lens can be adjusted directly to the focus range.In light of the principle that most natural scenes have certain invariability in power spectra,the fine focus adjustment evaluation function based on power spectrum is established and the accurate focus is further found.The simulation results have shown that this method which combines rough and fine focus adjustment can ensure the focusing accuracy while improving the focusing speed.