Ocean thermal energy conversion technology has limited applications due to low energy utilization efficiency. This study proposes a pre-expansion ejector absorption power cycle for ocean thermal energy conversion. To enhance the turbine output, a pre-expansion separator heated by discharged warm seawater, along with an ejector, was introduced between the generator and absorber. A thermodynamic analysis strategy for the proposed cycle was conducted using MATLAB and Aspen Plus. A performance comparison between the pre-expansion ejector absorption cycle, traditional ejection cycle, and non-ejection two-stage absorption cycle under the ocean thermal energy working conditions was conducted. Parametric analysis was performed to investigate the cycle performance. In addition, the working range of the pre-expansion ejector was analyzed. The results show that the ejection-driven turbine output work effectively increased by 79.38% with the pre-expansion process, although the thermal efficiency slightly decreased. When the temperatures of warm and cold seawater were 26–32 °C and 4–7 °C, the lower and upper boundaries of the pre-expansion pressure are 560 kPa and 680 kPa, respectively. When the pre-expansion pressure varied from 600 to 660 kPa, the entrainment ratio and turbine outlet pressure drop were 0.541–1.018 and 344.9–348.9 kPa, respectively. The pre-expansion pressure and temperature of the cold source influence the cycle performance in a more complex manner because there exists a working range for the ejector, which is dominated by the pre-expansion pressure and influenced by the temperature of the heat and cold source.