Abstract: In order to study and design a new type of bionic robotic fish with high softness and strong environmental adaptability, imitating the shape of shark and the swimming posture of tuna fish, a bionic robotic fish with hydraulic flexible driving structure is designed. In view of the problem that the single-joint hydraulically driven flexible robotic fish has a C-shaped swing posture that does not conform to the swing rule of tuna fish, a two-joint hydraulic flexible drive is used to simulate the S-shaped swing of the fish, and the internal structure of the bionic fish is designed according to the principle of the hydraulic flexible actuator. According to the theoretical wave equation, the swing amplitude of the robotic fish is determined, the magnitude of the pressure load applied inside the flexible actuator is calculated by numerical simulation, and the driving efficiency of the hydraulic flexible actuator is analyzed and calculated. The software of finite element analysis is used to simulate the autonomous swimming process of the robotic fish in the fluid. And the autonomous cruise process of the two-joint robotic fish and the one-joint roboic fish are simulated and compared to obtain the movement postures, swimming velocity and flow field of the two robotic fishes when they autonomously cruised in the fluid. The results show that at the same frequency and tail-fin swing, the average velocity of the two-joint soft roboic fish cruising is 0.29 BL/s, which is higher than the average velocity of the the one-joint roboic fish 0.15 BL/s. And frome the velocity vector diagram, it can be concluded that the S-type swing of the two-joint roboic fish is closer to the real fish swing attitude, and a series of discrete reverse Karman Vortex Streets will be generated during the movement, so the two-joint bionic fish has a higher propulsion efficiency.