Unmanned underwater platforms are commonly used in ocean exploration and development to help researchers understand underwater conditions. A Model Reference Adaptive PID (MRAC-PID) control method is designed to solve the problem that a small unmanned underwater platform can’t carry different equipment to maintain motion and attitude stability in the harsh seabed environment. In this paper, the kinematic and dynamic system model of the platform in the stable state is set as the reference model of the control algorithm. When the model parameters of the controlled object change due to the environment or its factors, the controller will compare the control difference between the reference model and the controlled object. At the same time, the tracking control error will be fed back to the adaptive adjusting mechanism in real-time, so that the controller keeps iterating the control parameters. Through continuous optimization of control parameters, the control effect of the controlled object and the reference model can be kept consistent at all times so that the attitude of the unmanned underwater platform can be kept stable at all times. Compared with the traditional PID and fuzzy PID control algorithm, the algorithm developed in this paper improved the control performance indicators greatly. Even though the system model structure and parameters change with the operating environment, the MRAC-PID control algorithm is still effective and shows a very good robustness. The proposed control approach is also suitable for real-time application, which can provide a reference for the subsequent related researches.