An intake system-connected mechanical supercharged four-stroke internal combustion engine which relates to the technical field of internal combustion engines comprises air cylinders, pistons, connecting rods, intake mechanisms, intake valves, exhaust mechanisms, exhaust valves, connecting pipes and a crankshaft, both ends of the third connecting pipe are respectively connected with the second intake mechanism and the fourth exhaust mechanism, both ends of the first connecting pipe are respectively connected with the first intake mechanism and the third connecting pipe, and both ends of the second connecting pipe are respectively connected with the third exhaust mechanism and the third connecting pipe. In the process of operation, the third piston and the fourth piston can utilize the mechanical energy of the first piston and the second piston by means of the crankshaft-connecting rod mechanism; by controlling the timing of the intake valves and the exhaust valves, the sucked air can be supercharged by the third air cylinder and the fourth air cylinder, consequently, the first air cylinder and the second air cylinder can suck more air, and the effective power of the engine can be increased. The design of the intake system-connected mechanical supercharged four-stroke internal combustion engine is reasonable, the structure is simple, and the invention is applicable to the mechanical supercharging systems of four-stroke engines.