In order to effectively solve the problem that is difficult to detect for laser spot in underwater laser communication system, a simple and practical underwater laser spot detection algorithm is proposed for the low visibility and complex underwater environment where the water attenuation coefficient is about 5.5 dB/m and the emitter is 5-10 m away from the acquisition end. Firstly, the algorithm enhances the contrast between the target spot and the background through the image defogging operation, after Gaussian filtering, the image noise is eliminated. Then the appropriate threshold is selected for binarization to extract the target laser spot. Finally, the spot center is calculated by the improved gray center of gravity algorithm. The experimental results show that the algorithm is not only simple in calculation, but also can accurately identify the target laser spot and obtain its central coordinates in the case of water quality with low visibility and high attenuation, so that the underwater laser communication system can adjust according to the spot position and realize normal communication.