Abstract: Currently, underwater robots are used in the fields of exploration, monitoring, search & rescue, and maintenance in ocean development. As a multi-joint and highly flexible underwater robot, the underwater snake robots are superior to traditional underwater robots in terms of motion efficiency, function execution, environment adaptation, and autonomous learning. For the motion efficiency problem of underwater snake robots with a rear thruster, an efficient motion mode combining lateral undulation motion and thruster propulsion is proposed. Firstly, the dynamic model of underwater snake robot is established. Then, the motion efficiency of the snake robot is evaluated by the transportation economic metric method. Based on this evaluation method, the NSGA-Ⅱ (non-dominated sorting genetic algorithm Ⅱ) is used to optimize four kinematic parameters, including three lateral undulation gait parameters (the amplitude of joint motion, the frequency of joint motion, the phase shift between the joints) and the thruster force. The optimization results of the three motion modes show that: in the low-speed segment, the lateral undulation mode is of the highest efficiency; in the medium speed segment, the hybrid motion mode combining lateral undulation motion and thruster propulsion, is of the highest efficiency, and in this mode, not only the speed is much faster than that of the lateral undulation mode, but also the overall motion efficiency is higher than that in the thruster mode; in the high-speed segment, the thruster mode is of the highest efficiency and the fastest forward motion speed. Finally, the effectiveness of the proposed motion mode is verified by pool experiments. It maximizes the movement ability of the underwater snake robots with thrusters, improves the movement efficiency of the robot, and elongates the battery life.