In optoelectronic images of fixed large visual field, the angular velocity of space target in long distance is slow and its image is small, which make it difficult to be detected among so many stellar images. In order to solve this problem, a detecting method for slow small target is put forward based on time-space difference fusion after stellar image registration. In this method, the morphological Top-hat operator is adopted firstly in space domain to suppress the background and enhance the target. Then, the stellar registration and difference fusion are done in time domain to enhance the moving target. Finally, the target can be confirmed through the adaptive segmentation and track correlation. The test result shows that by considering the target’s characteristic in time and space domain, the method mentioned above can effectively detect the slow small target with low Signal-to-noise Ratio (SNR) in complicated background and own good real-time performance.