With the development of military technology, submarine is becoming the key to sea field. It is outstandingly important to research communication system with high data rates and safety for improving the survival ability of submarine. The traditional communication means, such as VLF and RF, need submarine float in the sea surface, which is inconvenient in war. Blue-green laser communication has attracted considerable attention as the "window" for underwater communications. In this paper, the down link communications performance of satellite-submarine is based on single photon detection. The downlink model is analyzed, and the factors of transmitted laser power, sea water working depth, pointing and tracking errors are all discussed. The results show that GM APD based single detection communication systems are available for submarine working in depth of 30 -500 m. However, there are still more key technologies to research before the realization of blue-green laser communication between satellite and submarine.