An oil injection system of an engine comprises an oil supply passage (21) communicated with an oil pan (2) and a crankcase (1). Lubricating oil enters the crankcase (1) through the oil supply passage (21). One end of the oil supply passage (21) is provided with an oil injection port (210), and the oil injection port (210) is formed in a sidewall of a cylinder (8) and located below a lower dead point of a piston (7). With the oil injection system of the engine, cylinder walls of the engine and the crankcase can be sufficiently lubricated, and meanwhile, a passage from a lubricating oil tank to a crankshaft is simplified, so as to facilitate the processing and manufacturing.