The invention relates to an engine body of a large-travel low-emission single-cylinder air-cooled diesel engine. A crankcase containing cavity, an air cylinder sleeve, a left cooling fin connecting board, a right push rod chamber, cooling fins, an air guide cover locating frame, a side cover installation face, an air cylinder cover installation face and a start motor installation hole are arrangedon the engine body of the diesel engine. The cooling fins are arranged on the outer wall of the air cylinder sleeve in the axial direction of the air cylinder sleeve, and the height size from the center of a crankshaft bearing installation hole to the air cylinder cover installation face is 171 mm. The ten cooling fins are arranged on the outer wall of the air cylinder sleeve in the axial direction of the air cylinder sleeve. The area of the cooling fins on the periphery of the air cylinder sleeve installation engine body is increased, the periphery of the air cylinder sleeve of the diesel engine of the type is cooled more evenly, the deformation condition of the air cylinder sleeve is effectively reduced, engine oil consumption is reduced, and particle emission is lowered.