The type selection, design and experiment of a built-in labyrinth oil separator for a new type of six-cylinder natural gas engine were conducted. The experiment results show that at full load and full speed, the temperature drop of a labyrinth oil separator is about 7 ℃. The temperature drop of a closed type of labyrinth oil separator is about 5 ℃ lower than that of an open type under the same amount of crankcase blow-by. The internal pressure and separation efficiency of a labyrinth oil separator increase with the amount of crankcase blow-by. The closed and open separation systems can achieve separation efficiency of about 87% at the amount of crankcase blow-by of 180 /min. When the amount of crankcase blow-by is less than 180 /min, the separation efficiency of the closed type is higher than that of the open type with a maximum margin of about 10%. The results also show the key factors that affect the separation efficiency of a labyrinth oil separator are the optimized combination of structure size and installed distance of internal diaphragm separators and dampers, amount of crankcase blow-by, and selection of ventilation type for an open or closed oil separation.