Currently, the thermal efficiency of vessel diesels only reaches 48∼51%, and the rest energy is rejected to the environment in forms of exhaust, cooling water, engine oil and so on. Meanwhile, energy is required when generating electricity and fresh water that are necessary for vessels. A system that combines the ORC thermal electric generation system with the single-effect evaporating desalination system simultaneously driven by waste heat of charge air is proposed. The research object was 12S90ME-C9.2 diesel engine produced by MAN corp., and a calculation model of the system is built by MATLAB. The variation of the output power, the thermal efficiency and the freshwater production with some operational parameters of the combined system are calculated and analyzed. On the other hand, under the condition of an assumed freshwater production 110.3t/d, the variation of the charge air temperature at the outlet of the desalination evaporator with some operational parameters of the system is studied in the paper.