This paper presents a review of the research on closed thermodynamic cycles of ocean thermal energy conversion (OTEC) system, including a description of thermodynamic cycles with either pure or mixture working fluids, and describes the effects of various working fluids on cycle efficiency. For cycles with pure working fluids, the efficiency changes due to change in the evaporation and condensation temperature caused by heat resource differences. For cycles with mixture working fluids, the efficiency may be improved by a number of techniques, such as heat recovery of ammonia-depleted solution and the intermediate extraction regeneration. Furthermore, the effect of the ejector on performance of the cycle is also reviewed. Finally, the techniques used to improve efficiency are discussed and summarized. In general, the thermodynamic efficiency can be improved by adopting suitable working fluids and measures which could increase the utilization rate of ocean thermal energy. The related methods need to be compared and analyzed under the same working conditions to determine which is the most effective.