This paper proposes a high-efficiency R717 Ocean Thermal Energy Conversion (OTEC) power cycle with an expansion valve and a cooler. To provide the basic design data for the operating variables of this high-efficiency R717 OTEC power cycle, the cycle performance was analyzed in terms of the evaporation heat capacity, condensation heat capacity, total work and efficiency using the HYSYS program. The operating variables considered in this study included the high-stage turbine outlet pressure, the expansion valve outlet pressure, the turbine efficiency, the pressure drop of the evaporator and the cooler outlet vapor quality of the R717 OTEC power cycle. The main results can be summarized as follows. The high-efficiency R717 OTEC power cycle was most affected by the turbine efficiency and the cooler outlet vapor quality. Therefore, these effects must be considered when the R717 OTEC power cycle is designed. Furthermore, the actual application of the proposed OTEC power cycle requires the development of a highly efficient turbine, the optimal control of the cooler outlet vapor quality and the optimization of the system. The efficiency of the proposed OTEC system is compared with that of Uehara and Kalina one. The efficiency of Uehara, Kalina and proposed OTEC system is 2.379, 2.361 and 2.401, respectively. This OTEC system is the highest efficiency among other systems.