Aiming at the characteristics that the contrast of schlieren sphere image is weak, the color of sphere target is black, and the sphere target is surrounded by background area in the fast collimation process of integrated diagnostic system, a method for calculating the schlieren sphere center based on gray stretching and digital morphology is proposed in this paper. Firstly, the original image is transformed by linear gray scale to enhance the contrast between the schlieren sphere target and the background area, and then the linear gray scale image is binarized by using the OTSU method. Secondly, the background area is transformed into a continuous connected region by using digital morphological operations such as corrosion and expansion, and search for the maximal connected area is the background area. Thirdly, the image of using digital morphology method and the image of background area are processed NOR operation to obtain the schlieren sphere target. Finally, on the basis of edge detection by sobel operation, the least square method is used to fit the center and radius of the schlieren sphere. The experimental results show that the algorithm can calculate the center of the schlieren sphere whose gray level difference between the sphere area and the background area is greater than 10( the max gray level is 4095). The error between the center coordinate obtained by circle fitting method and the real center coordinate is less than 2 pixels, which meets the requirement of the integrated diagnostic beam fast automatic collimation system that the accuracy of the schlieren sphere center is less than 3 pixels.