To alleviate the consequences of the freshwater crisis and fully utilize marine resources in tropical coastal areas, this study presents a low-pressure flash evaporation desalination system driven by ocean thermal energy (OTE). Through gravity and atmospheric pressure, a natural vacuum can be formed in the evaporator, which achieves flash evaporation for warm surface seawater. Then, the vapour is condensed into freshwater via an inner coil condenser with flowing cold deep seawater. Using a water pump to fill the evaporator intermittently, the non-condensable gas accumulated in the desalination process can be thoroughly discharged. Based on the process of mass and heat transfer inside the system, a thermodynamic model was developed. Several performance evaluation indices, including water productivity, specific electrical energy consumption (SEEC) and recovery ratio (RR), were experimentally investigated under different parameters. The results indicate that productivity and SEEC are higher with increasing seawater flow rate and decreasing deep seawater temperature. Under a warm seawater temperature of 30 °C and a deep cold seawater temperature of 8 °C, the system obtained a maximum water productivity of 5.3 kg/h, and the corresponding SEEC and RR were 0.126 kWh/kg and 1.5%, respectively. Finally, compared with solar-driven desalination systems, the proposed OTE desalination system requires less energy consumption, which shows an attractive application prospect.