Ocean thermal energy conversion (OTEC) has attracted lots of attention for its potential in clean energy production. In order to further improve system efficiencies, a new type of OTEC system using two ammonia ejectors is proposed. Effects of working pressures and geometrical parameters on vapor-vapor ammonia ejectors are studied numerically. The results show that performance characteristics of ammonia ejectors are mostly in subcritical mode for the working conditions of OTEC, which is different from other refrigeration applications. Entrainment ratios of ammonia ejectors range from 0.44 to 1.68 based on the working conditions in the present work. Increases of area ratio and decreases of nozzle exit position are beneficial to the improvement of ejector entrainment ratios, while the performances of ejectors decrease with increasing primary flow pressure and with decreasing secondary flow pressure. The thermal efficiency of the new OTEC system decreases with increasing primary and secondary flow pressures, but increases with increasing entrainment ratios. Due to the enhancement of turbine output work and reduction of pumping power brought by the two ejectors, the highest thermal efficiency for the proposed OTEC system with ejectors is 3.95% which is about 10.33% higher than a normal OTEC system without ejectors.