Water depths are the basic data for surveying and mapping seabed topography, which is of great significance for marine scientific research. A diffractive optical system has the advantages of small size and light weight, and a conformal design can reduce the impact of the load on the aerodynamic performance of an aircraft, which is conducive to the formation of a large optical aperture. This paper introduces the system scheme and parameters of airborne bathymetry lidar based on a large-aperture receiving conformal diffractive optical system. The narrow bandwidth characteristic of the diffractive optical system is used to suppress the received background light noise. To realize both direct and coherent detection methods, the seawater depth detection of airborne lidar is analyzed based on the minimum detectable signal-to-noise ratio. The results demonstrate that when the receiving aperture is 0.6 m, the instantaneous receiving field of view is 50 mrad, the average transmitting power is 50 W, the detection depth of the system in the direct detection mode during the day is 69 m, and the detection depth in the coherent detection mode is 86 m.