Space observation image is easy to be interfered by cosmic rays. A rejection algorithm of cosmic ray in space observation image is presented. Firstly, the stray light is calibrated in observation image by Top-hat transform. After processing, the image background is uniform, and the histogram shape recovered nearly Gaussian. Then, a combination of morphological operations and median filter is used to eliminate the interference from bright saturated stars. Finally, the Laplacians edge detection algorithm is used to detect cosmic noise pixel, and the neighborhood minimum filtering method is used for cosmic ray pixel interpolation. The experimental results show that, the algorithm is superior to other algorithms in two aspects of the detection and false detection probability.