Based on the theory of MTF in optics, the features of VISSR optical imaging system are analyzed. A criterion and a method for VISSR focusing are proposed and the experiment and the results for the frequency analysis method using FFT through receiving stretcher digital images of the Japaness GMS-4 satellite are given. The tests show that the method developed in this paper would be useful and practical for VISSR focusing control of the geostationary meteorological satellite on the orbit.