Ocean thermal energy is formed by temperature difference between sea surface and deep sea. Underwater glider is a new kind of autonomous underwater vehicle. It ascends and descends between the sea surface and deep sea periodically by adjusting its net buoyancy. Thermal underwater glider can harvest ocean thermal energy through the utilization of phase change material (PCM) and convert it into mechanical energy for buoyancy-driven. In this paper, the fundamental principle of this thermal engine is unveiled. A nonlinear model for relationship between system pressure and phase change rate is established. Based on the established nonlinear model, influencing factors for system pressure and stored energy are analyzed comprehensively. Selected PCM is Hexadecane. Value range for air solubility in liquidus PCM is 0.06–0.1, and for residual air in the system is −0.02 to 0.05. The influence of these two factors on relative energy storage is similar. For typical values of air solubility 0.08 and residual air 0.005, utilization ratio of the thermal engine is less than 50%. Through experiment in the lake, the nonlinear model was verified, maximum system pressure was 12.5 MPa, and average stored energy of each cycle was 2.48 kJ. A prototype of thermal underwater glider has also been tested in the South China Sea. It had worked continuously for 29 days without any failure. Total number of working profile was 121 and total cruising range was 677 km, total stored energy was 300 kJ. High reliability and performance was validated by the sea trial.