Satellite-to-sea exploration has the advantages of large-area synchronous observation,full-day operation,and no restrictions on territorial waters and airspace.The laser link loss of satellite-to-sea observation was analyzed,and the energy transmission model of the spaceborne laser underwater target detection was built,and the corresponding limit detection depth based on single-photon detectors was discussed.The results show that in the case of the first class sea area and 200 km orbital height,the maximum detecting depth achieves 212 m to the underwater targets with an equivalent diameter of 2 m.Therefore,it is possible to detect the deep-sea underwater targets by spaceborne laser in the medium or low orbit.This research provides a support for further development of the deep-sea underwater target detection by spaceborne laser.