On the bests of the airborne blue-green laser depth-measuring Pulse transmiSSion Process and treating the laser light paths propagating downward and upward the sea as different transmission processes, an airborne laser bathymetry lidar equation is obtained through the Phenomenological model of beam spreading in ocean water and the underwater finite source distant law. The depth-measuring capability of a bathymetry lidar is accurately calculated and the factors affecting the depth-measuring caP8bility of a bathymetry lidar are analyzed.