A cold start mechanism for an engine comprises a piston and a spring which are integrated in a rocker arm of the engine. A piston has a non-operation position and an operation position in a piston hole in the rocker arm, and the spring and fluid enable the piston to move between the operation position and the non-operation position. At the operation position, the part, in the piston hole of the rocker arm, of the piston enters an annular groove in a rocker arm shaft, and the rocker arm and the rocker arm shaft are in mechanical connection; the rocker arm is prevented from rotating on the rocker arm shaft, and an air valve of the engine is made not to fall and to be kept open. At the non-operation position, the piston in the rocker arm and the rocker arm shaft are disengaged, no effect is achieved, the rocker arm can rotate freely on the rocker arm shaft, and the air valve of the engine returns to be in a normal working state. According to the cold start mechanism for the engine, the simple and reliable mechanism and the novel and unique design are adopted, and the air valve of the engine is kept open when the engine flames out or stops or starts, so that the performance, particular the stopping, starting and idling performance, of the engine is improved, and the oil consumption and emission of the engine are reduced.