The invention discloses a combined desulphurization and denitration device and method of a marine diesel engine. The exhaust gas of the diesel engine is divided into two paths, wherein one path enters a denitration channel A and is recirculated to an air cylinder to participate in burning so as to lower the NOx emission of an engine; the other path enters a desulphurization channel B and is discharged to ambient atmosphere after being subjected to desulphurization processing. In the denitration channel A, the exhaust gas flows through components including a one-way stop valve, a Venturi scrubber, a cooler, a cleaning device, a blower and the like, particles and SO2 in the exhaust gas are removed through processing processes including pre-washing, cooling, washing, pressurizing and the like, and finally, the exhaust gas is mixed with fresh air from the intake channel of the diesel engine to enter the air cylinder and participate in burning; in the desulphurization channel B, the exhaust gas is discharged into the environment atmosphere after being subjected to particle removal by washing tower desulphurization. The Tier III requirement of IMO (International Maritime Organization) on marine fuel sulphur content and NOx emission in the exhaust gas can be met, and meanwhile, the corrosion and the abrasion on an engine compressor, a cylinder liner and the like by the exhaust gas can be lowered.