With the upgrading of emission regulation, engine closed crankcase ventilation system become more and more popular, but the oil mist and PM of crankcase ventilation can contaminate engine parts and even affect engine performance. In order to reduce the effect of ventilation pollutants, a two-stage oil-gas separator system was adopted on an in-line 4-cylinder diesel engine, in which a pre-separator was integrated in the cylinder head cover and an external separator was connected in series. The oil carryover test of two-stage crankcase ventilation was conducted on an engine test bed and was compared with that of open crankcase ventilation. The results show that the twostage oilgas separator system can make the oil carryover within 0.106 g/h with above 92% separation efficiency of external separator, which can meet the design requirements. Compared with open crankcase ventilation, the BSFC, NO_x and PM emission of NRTC for closed crankcase ventilation increase 0.4%, 1% and 3.4% respectively and the steady specific NO_x emission almost keeps unchanged, which have little influence on engine performance.