The present invention discloses an energy-saving and emission-reducing multistage throttling expansion method for engine. In a crevice passage disposed between the combustion chamber and the crankcase, a multistage throttling is disposed for converting pressure energy of the high-pressure blow-by gas into kinetic energy and momentum, and a multistage expansion is disposed for expanding and dissipating the incoming kinetic energy and momentum of the high-velocity blow-by gas into heat, so that to realize the multistage throttling and expansion method, reduce the leaking of the unburned fuel-air mixture and the burned gas, the hydrocarbon emissions hidden in the intra-cylinder carbon deposition and exhaust gas emissions of the engine, and also improve the engine efficiency and the overall performance of the engine.