Based on an equivalent air-fuel ratio natural gas engine, the influence of full flow equipment operation state on crankcase pressure in closed crankcase ventilation system was studied. Flow of CVS system had no effect on crankcase pressure, but the intake pressure was positively correlated with the crankcase pressure. Therefore, measures such as turning off the intake air conditioning system or using an open system should be taken to avoid the introduction of interference for measuring the crankcase pressure. It's difficult to maintain negative pressure inside crankcase during WHTC cycle by using the negative pressure of supercharger inlet. Therefore the crankcase ventilation system could be optimized by combining the introduction of negative pressure at the back of throttle with the active oil-gas separator. With the optimized system, the crankcase pressure could be maintained between －2.1 kPa and 0 kPa and the average crankcase pressure was 1 kPa lower than the previous, which could meet the requirements of China Ⅵ legislation.