A decompression device for an internal combustion engine comprises a valve rocker shaft bracket and a valve rod. A valve spring is sleeved on the valve rod, and above the valve rod a valve rocker is arranged. The valve rocker shaft bracket is connected to a decompression bracket. The valve rocker is movably connected to a valve opening/closing device connected with a control device. When the internal combustion engine is started, the valve opening/closing device receives a control command from the control device to keep connecting with the valve rocker for a time, and then disconnect, so that the valve rod arrives at the valve closing position.