A combination of formula derivation and simulation optimization was used to design a LiDAR optical-mechanical system for measuring water depth. The light source of this system used 532 nm and 1 064 nm dual-frequency lasers. Three receiving channels of 532, 647 and 1 064 nm were designed. The planned flight height was 140-500 m, the variable scanning angle is 9°-15°, the divergence angle was less than 0.5 mrad, and the surface point density range was about 0.687-4.170 points/m2. In this paper, Raman band was designed as a water depth measurement band to improve the measurement effect in shallow water. The variable scanning angle was used to realize the functions of variable resolution under a fixable width and the consideration of high precision and large field of view to adapt to different application scenarios.=