13 the operation of the electromagnet 9 is controlled by a time delay relay 10 and when the internal combustion engine is stopped the decompression cushion block 6 is located between the decompression shaft 3 and the valve rocker 5 when the internal combustion engine needs to be stopped the time delay relay 10 controls the electromagnet 9 the time delay relay 10 switches on the circuit of the electromagnet 9 the movable iron core 8 propels the shifting fork shaft 13 to move towards the direction of the valve rocker 5 the shifting fork 14 compresses the springs 4 when the valve rocker 5 drops the decompression cushion block 6 is located between the valve rocker 5 and the decompression shaft 3 the circuit of the electromagnet 9 is switched off under the acting force of the valve spring 12 the decompression cushion block 6 is located between the decompression shaft 3 and the valve rocker 5 and cannot be reset the valve rod 11 cannot arrive at a valve closing position and the valve is at an open state when the internal combustion engine needs to be started a signal is sent to the time delay relay 10 at first so that the position of the decompression cushion block 6 is maintained between the valve rocker 5 and the decompression shaft 3 the power supply of a starter is turned on the starter drives a flywheel to rotate a crankshaft of the internal combustion engine reaches a starting speed the time delay relay 10 switches off the circuit of the electromagnet 9 when the valve rocker 5 moves.