An engine with decompression devices, comprising: a cylinder head, provided with a camshaft (31,131), an intake valve, an exhaust valve, an intake rocker arm, and an exhaust rocker arm, the camshaft (31,131) includes a valve cam; a crankshaft, configured to rotate together with the camshaft (31,131); an integrated starter and generator, configured to rotate together with the crankshaft; wherein the integrated starter and generator is arranged at one end of a power output shaft connected with the crankshaft, and is located at one end of the crankshaft; a control unit, provided for controlling the integrated starter and generator; and two decompression devices, arranged on the camshaft (31,131), including a one-way decompression device (30,130) and a centrifugal decompression device; wherein the one-way decompression device includes a one-way clutch, a decompression cam, a push member, and a resilient member; and the one-way clutch is sleeved to the camshaft; the decompression cam is fixedly arranged on circumference of the one-way clutch and is provided with a stop portion; the push member includes a driving portion and a blocking portion; and the resilient member pushes the push member and either the intake rocker arm or the exhaust rocker arm; wherein the centrifugal decompression device includes a sprocket, a centrifugal block, a decompression axle, a push rod, and a pre-force member; the sprocket and the valve cam are fixedly arranged on the camshaft; the centrifugal block is pivotally arranged on the sprocket; the decompression axle is arranged passing through the camshaft; the push rod slides in the valve cam; and the pre-force member pushes against the centrifugal block and the sprocket, respectively; wherein, when the crankshaft and the camshaft turns reversely, the decompression cam drives the push member such that the driving portion pushes against a driven portion of the rocker arm, and the rocker arm turns and pushes the valve to open for a first tiny valve lift and thus causing an engine decompression effect; however when the camshaft turns forward, the stop portion of the decompression cam abuts against the block portion of the push member, and as such, the engine is not subject to a status of decompression, and the centrifugal decompression device, under a specific rotation speed of the crankshaft, acts on either the intake rocker arm or the exhaust rocker arm such that either the intake valve or the exhaust valve is opened for a second tiny valve lift. a cylinder head, provided with a camshaft (31,131), an intake valve, an exhaust valve, an intake rocker arm, and an exhaust rocker arm, the camshaft (31,131) includes a valve cam; a crankshaft, configured to rotate together with the camshaft (31,131); an integrated starter and generator, configured to rotate together with the crankshaft; wherein the integrated starter and generator is arranged at one end of a power output shaft connected with the crankshaft, and is located at one end of the crankshaft; a control unit, provided for controlling the integrated starter and generator; and two decompression devices, arranged on the camshaft (31,131), including a one-way decompression device (30,130) and a centrifugal decompression device; wherein the one-way decompression device includes a one-way clutch, a decompression cam, a push member, and a resilient member; and the one-way clutch is sleeved to the camshaft; the decompression cam is fixedly arranged on circumference of the one-way clutch and is provided with a stop portion; the push member includes a driving portion and a blocking portion; and the resilient member pushes the push member and either the intake rocker arm or the exhaust rocker arm; wherein the centrifugal decompression device includes a sprocket, a centrifugal block, a decompression axle, a push rod, and a pre-force member; the sprocket and the valve cam are fixedly arranged on the camshaft; the centrifugal block is pivotally arranged on the sprocket; the decompression axle is arranged passing through the camshaft; the push rod slides in the valve cam; and the pre-force member pushes against the centrifugal block and the sprocket, respectively; wherein, when the crankshaft and the camshaft turns reversely, the decompression cam drives the push member such that the driving portion pushes against a driven portion of the rocker arm, and the rocker arm turns and pushes the valve to open for a first tiny valve lift and thus causing an engine decompression effect; however when the camshaft turns forward, the stop portion of the decompression cam abuts against the block portion of the push member, and as such, the engine is not subject to a status of decompression, and the centrifugal decompression device, under a specific rotation speed of the crankshaft, acts on either the intake rocker arm or the exhaust rocker arm such that either the intake valve or the exhaust valve is opened for a second tiny valve lift.