In this paper theoretical analysis of an ocean thermal energy conversion (OTEC) system was conducted using the Rankine cycle based on the first law of thermodynamics and a mathematical model of components of the system was established. Efficiencies of six types of working fluids were evaluated and compared under the uniform conditions. Finally a 15 kW OTEC plant using the Rankine cycle was constructed and change rules of the thermal cycle efficiency were obtained with various parameters. The results show that by using the Rankine cycle R717 is “the most suitable” for an OTEC system among the selected working fluids. Thermal cycle efficiency initially increases and then decreases with increasing turbine inlet pressure; maximum thermal cycle efficiency was achieved when the turbine inlet and outlet temperature are fixed. Thermal cycle efficiency was 3.2% when the turbine inlet pressure was 0.85 MPa under calculating the conditions. The thermal efficiency decreased with an increase of the condensing temperature under the same turbine inlet pressure. The consistency between the experiment and theoretical results was verified; the experimental value was lower than the theoretical. Meanwhile, this work can provide design data for an OTEC plant and operating experience for an OTEC system was obtained.