Autonomous underwater vehicles (AUVs) play an important role in the ocean observation and marine scientific research. The utilization of marine environmental energy is a desirable way to enhance the sailing range and endurance of AUVs. In this paper, a detailed model with four sub-models is established to describe the ocean thermal energy utilization process in AUVs, where the factors of system pressure, material's thermal-pressure-physical properties and grid nodes are considered. Based on the model, performance of the utilization system under constant and variable temperature boundaries is analyzed. At last, an AUV driven by ocean thermal energy is deployed in South China Sea to test the utilization performance, and the model is validated by the practical trail data. It is proved that thermal-driven AUV saves about 50% energy of buoyancy-driven process in sea trail. The simulation results are in good agreement with the trial data and prove the correctness and effectiveness of the model. The utilization system shows a better performance in the temperature condition with a higher surface temperature and a greater gradient of thermocline. When reaching the designed depth, AUVs can obtain plenty of time for phase change. The conclusions drawn in the paper provide some guidance for development of the thermal-driven AUV.