Optical imaging has been widely used in many fields as a tool for acquiring images, which can be used in microscopic observation, medical analysis, remote sensing, and astronomical observation. In the field of astronomical observations, it is very important to adjust the focus of the coronagraph. Due to the significant differences of the intensity between the corona and the solar photosphere, the ground-based coronagraph usually adopt an occulter, which is a little larger than the solar disk, to shield the light from the solar photosphere. In absence of the Sun as reference, focusing of the coronagraph is much harder than usual optical system. For the ground-based coronagraph, we developed a method of focusing using the solar disk image behind this disk occulter. Based on a series of images collected by shifting the center of the solar disk, we extract edge information of the solar disk and use the edge gradient algorithm to fit the focal length. This method can reduce the error of manual focusing. We can precisely find out the coronagraph focal point and obtain a clear coronal image, which lays a foundation for the technical support aboutremote control system of the coronagraph.