An engine includes an oil pan used for storing lubricating oil, a crankcase, a cam box, a rocker, and a distribution chamber used for separating oil & gas mixture into oil mist and liquid oil. A lubricating system comprises: the oil pan and the crankcase are communicated through an oil supply passage, and a one-way valve is disposed in the oil supply passage; the crankcase and the distribution chamber are communicated through a first oil conveying passage, and a one-way valve is disposed in the first oil conveying passage; the distribution chamber and the cam box are communicated through a second oil conveying passage; the cam box and the rocker chamber are communicated through a third oil conveying passage; the rocker chamber and the crankcase are communicated through at least one oil return passage, and a one-way valve is disposed in the oil return passage.