Focusing technique of space camera has relation to camera image quality, being one of the key technologies of on- orbit test of spacecamera. Image based focusing technique with simple structure is highly intergrated, which is used in space camera. The key of Image based focusing is to estabilish focusing evaluation function. Focusing evaluation function can provide guidance for the focusing process. This paper discusses the influence caused by defocus. Based on the study on focusing evaluation function methods of foreign space camera, some instructional conclusions about choosing focusing evaluation function method and advantages and disadvantages are obtained, which provide reference and help for space camera focusing technique.