The invention relates to a gas inlet system independent type mechanical supercharged four-stroke internal combustion engine, belonging to the technical field of internal combustion engines. The four-stroke internal combustion engine comprises cylinders, pistons, connecting rods, gas inlet mechanisms, gas inlet valves, exhaust mechanisms, exhaust valves, connecting pipes and a crankshaft, wherein the gas inlet mechanisms and the exhaust mechanisms are mounted at the top ends of the cylinders, the two ends of the first connecting pipe are respectively connected with the first gas inlet mechanism and the fourth exhaust mechanism, and the two ends of the second connecting pipe are respectively connected with the second gas inlet mechanism and the third exhaust mechanism. During the working process, the third piston and the fourth piston can utilize the mechanical energy of the first piston and the second piston through a crankshaft-connecting rod mechanism; and by controlling the timing of the gas inlet valves and the exhaust valves, the third cylinder and the fourth cylinder can supercharge the air inhaled by the third cylinder and the fourth cylinder, and then the first cylinder and the second cylinder can inhale more air, thus the effective power of the engine is improved. The four-stroke internal combustion engine disclosed by the invention has the advantages of reasonable design and simple structure, and is suitable for mechanical supercharging devices of four-stroke engines..