A kind of method of measuring three-dimensional surface, gray gradient method, to use gray gradient of CCD image, is proposed. To use mid-variable skillfully, the mapping relation between gray gradient of CCD image and gradient of focused image surface(FIS) has been found, and the gray restricting equation has been changed into solvable monadic equation to find depth information on the FIS. By the restriction of geometry optics and space conjugate symmetric relation between focused image surface and object, the FIS is changed into trim size, so the aim to measure three-dimensional is turned into reality. Finally, the error factors affecting the system have been analyzed. The method overcomes the defect that traditional light-sectioning method is low efficient because of the tedious number of images, and its restricting conditions are not rigorous. The experiment with sphere and cylinder, error ratio of which is 6.0% and 4.85% respectively, shows the method is effective in the definite range.