The utility model relates to a single-cylinder double-piston internal combustion engine and solves the problem that the prior internal combustion engine has low energy conversion rate. The single-cylinder double-piston internal combustion engine comprises an air cylinders; the middle of the air cylinder is provided with an air distributing mechanism, a spark plug or a fuel oil pump; the air distributing mechanism and the spark plug are arranged annularly; two pistons are arranged correspondingly at two sides of the air cylinder. The pistons are connected with an output shaft through a transmission device. The pistons at two sides are connected with a central shaft respectively through symmetrical connecting rods, a crankshaft and other transmission parts, or one piston at one side is connected with the crankshaft through a piston, and the other piston at the other side is connected with the connecting rod at the front side through a rocker arm and other transmission mechanisms. The utility model has the advantages of high exhaust efficiency, energy saving, one time increase of the absorbed energy top area of an ignition plug when igniting and doing work, high energy conversion efficiency, strong power, little consumption of fuel and simplification of the air distributing mechanism, and is suitable for a multi-cylinder large-power internal combustion engine.