The utility model discloses a marine high-power medium-speed diesel engine. The engine comprises a cylinder assembly and an oil injection assembly, the cylinder body is a V-shaped cylinder body, the included angle between two columns of cylinders of the V-shaped cylinder body is 50 degrees, the hole diameter of cylinder holes of the cylinder body is 390 mm, the stroke of the cylinder is 550 mm, the center distance between the cylinder holes is 840 mm, and the rated power of a single cylinder is 850 kW. Split type air cylinder cover. The fuel injection assembly comprises a machine-mounted fuelpump, a fuel injection pump, a high-pressure fuel pipe, a fuel injector and a distribution block. The distribution block distributes high-pressure fuel of the machine-mounted fuel pump to the high-pressure fuel pipes on the sides of the two columns of air cylinders according to needs. According to the V-shaped cylinder body structure of the diesel engine, the engine body structure is more compact,the split type cylinder cover is convenient to maintain, parts are convenient to replace, and the split type pressure storage rail enables the oil injection pressure to be more stable.