Traditional hydraulic robot joints are usually large, because their actuators are controlled by separate servo valves. To reduce the size of robot joints, this study proposes a water-hydraulic rotating angle self-servo robot joint actuator with small size and high output torque by integrating with a control valve. A mathematical model of water hydraulic rotating angle self-servo robot joint actuator is established firstly based on its structure and working principle, followed by deriving and linearizing a governing equation. Then, an AMESim model of the robot joint actuator is established, and simulation is conducted to validate the proposed model. The results show that the input angle amplitude, external loads and water pressure have influence on the dynamic characteristics of robot joint actuator. Besides, when the robot joint actuator is optimized, it has smaller position following error and quicker velocity response.