The technology of ship flue gas desulphurization system is introduced, and the real ship retrofitting design of open flue gas desulphurization system is made. The main points and difficult points in the design are analyzed, and the notices and countermeasures in the design are proposed to meet the system function requirements. Through the test feedback in the real ship operation, the system can completely meet the approved emission value stipulated by the IMO, which provides the reliable basis for the retrofitting design of ship open desulfurization unit in the future.