The invention provides a natural gas engine with a treelike plate type self-adaptive ventilation system. The natural gas engine comprises an engine body and the ventilation system arranged on the engine body. The ventilation system comprises a primary separation device, a pre-separation device and a gas drive type oil-gas separator fixed to the engine body, wherein the primary separation device and the pre-separation device are arranged on a mounting groove in a gas cylinder cover on the engine body. The primary separation device is composed of at least two treelike plates. A gas inlet hole used for receiving an oil-gas mixture in a crankcase is formed in the mounting groove of the gas cylinder cover. An exhaust hole used for exhausting separated waste gas is further formed in the gas cylinder cover. A gas outlet pipe connected with a waste gas inlet of the gas drive type oil-gas separator is arranged on the exhaust hole. By means of the natural gas engine, passive separation and active separation can be achieved at the same time, and a ventilation closed cycle can be formed; and in addition, the flow speed can be self-adaptively controlled according to the pressure in the separation devices, the separation efficiency can be improved, and the requirements of CN6 engines can be met.