A gear type diesel engine relates to a diesel engine, and comprises an engine case, a rotor type piston ball, a rotor type air cylinder, a transmission gear set and an air supercharging device, wherein the rotor type piston ball and the rotor type air cylinder are positioned in the engine case; the transmission gear set and the air supercharging device are positioned on the outer side of the engine case; the rotor type piston ball and the rotor type air cylinder are meshed with each other, and mounted in the engine case through gear shafts; one end of each gear shaft extends out of the engine case and is connected with the transmission gear set; the air supercharging device is mounted on the outer side of the engine case; a flue gas exhaust pipe, a flue oil conveying pipe and an air inlet pipe which are communicated with the rotor type air cylinder are mounted on the engine case; an air inlet end of the air inlet pipe is connected with the air supercharging device. The gear type diesel engine has the advantages that the four strokes of air suction, compression, outburst and exhaust are completed during the process that the rotor type piston ball and the rotor type air cylinder are continuously meshed and separated, the operation is more stable, the noise is low, the structure is simple, the self weight is reduced, and the manufacturing difficulty and the manufacturing cost of the diesel engine are reduced.