In this paper, an underwater robot for inspecting and maintaining nuclear reactor vessel is developed. Based on the kinematics analysis of the robot, the workspace is obtained. In order to evaluate the influence of water environment to the robot, the hydrodynamic forces including water resistance and additional mass force are analyzed. Then some experiments including watertight test are performed to evaluate the performance of the robot and simulations are carried out to verify the theoretical analysis. The experimental and simulation results show that the developed underwater robot which can work at 22m in water meets the design objective basically and the analyses on kinematics and hydrodynamic forces are reasonable.