Ocean thermal energy refers to the thermal potential energy produced by the temperature difference between the warm surface seawater and the cold deep seawater. In this paper, a siphon flash evaporation desalination system using ocean thermal energy is proposed. Because it can utilize the ocean thermal energy directly for desalination, siphon flash evaporation desalination system has relatively higher energy efficiency compared with converting ocean thermal energy into electric energy and then using electric energy for desalination. The working principle of this system is introduced firstly. Then, the exergy, exergy loss and exergy efficiency in the flash evaporation, condensation and the whole system are carried out quantitatively. The results show that the exergy efficiency of the system which directly utilizing ocean thermal energy for desalination reaches to 7.81% under design conditions; lower surface seawater temperature, higher deep seawater temperature and higher flash temperature can result in an increasing of system efficiency, while the whole energy consumption shall also be taken into consideration. Then the simulation model of the whole system is created in ASPEN PLUS in order to investigate the influence of some most important parameters, such as surface seawater temperatures, deep seawater temperatures and difference of inlet temperature between surface and deep seawater. Finally, an experimental platform is established based on the working principle and process to verify the validity of the working principle and the simulation model. The siphon flash evaporation desalination system provides a novel method of direct high efficient conversion and utilization of ocean thermal energy and this work can provide the theoretical support for the feasibility of similar engineering applications.