Abstract: A snake-robot has many joints and degrees of freedom, and the underwater environment is highly complex and nonlinear, so it is very difficult to verify, analyze and optimize three-dimensional underwater gaits of a snake robot through experiments. To solve these problems, a simulation system for underwater locomotion is established based on the snake-robot prototype "Perambulator III". Firstly, the hydrostatics of snake robot in different positions and orientations is analyzed, the added-mass forces, linear and nonlinear terms of the viscous drag force are calculated, and effects of fluid torques on movement of underwater robot are studied. Then, the mechanical model about interaction between robot and water is established based on Morison equation. Finally, the module by module dive gait is simulated in the simulation system, the movement performance of the snake-robot is analyzed, and the corresponding experiments are made. The forward speed, the diving speed, and the movement trend are compared, results show that the mechanical model can accurately simulate the interaction between the water environment and the robot, and the simulation system can be used for verification and analysis of the snake robot's underwater 3D gaits.