Abstract: The lightweight small underwater vehicle which runs near free-surface of water is subject to wave disturbance which has a remarkable effect on the stability and precision of the control system. In order to suppress interference, a H_∞ mixed sensitivities controller was designed to adjust the longitudinal plane control system based on a dynamic model of the longitudinal plane of the small underwater vehicle. The wave force model was established. With the model, the mathematical and hardware-in-the-loop simulations of the longitudinal motion control of the small underwater vehicle were done. Test results show that the settling time of the controller is short, the overshoot of the system is small, and the system is robust. The wave disturbance can be suppressed. The results provide a very useful reference for the design of the whole control system of the underwater vehicles.