Power technology has long been the main problem that has plagued the realization of ocean exploration by autonomous underwater vehicles (AUVs). This paper introduces a new wave energy conversion device for AUV, which is sealed inside a closed floating body to avoid interaction with the marine environment. The system uses the gyroscopic effect to continuously convert the pitching motion of waves into electrical energy through flywheel rotation, and thus theoretically extend the endurance time of AUVs. In this paper, a mathematical model of the power generation device is established, and the effects caused by different parameters on the system behavior and energy output are analyzed. In order to reduce the cost of experiments, the energy conversion device is installed on an experimental platform that can simulate wave motion to observe its energy generation performance. The experimental results show that the established mathematical model can accurately reflect the real behavior of the power generation device on the platform under different wave conditions, and the energy output error is only 9.91%.