On the basis of numerical processing technology of image grayscale and binarization , the concepts of crack rate and calculation method on expansive rock are presented. The dry-wet cycle tests were carried out in situ at Luwangfen test section of South-to-North Water Transfer Project Central Route, the tests focused on the observations of development changes in cracks and soil moisture changes. According to the results in the tests, the variation law of crack width on expansive rock slope and its relationship with moisture content during the dry  wet cycle test were quantitatively analyzed, also the variation law of crack ratio was investigated. The development of crack on expansion rock is closely related to changes in the weather, and with the development of wet and dry cycles, the width of cracks and crack ratio will gradually increase. By using the Mat lab digital image processing method, the crack ratio can be used to effectively describe the growth and development of cracks on expansive rock.