The invention designs a self-detecting method of an oil-gas separation device based on a reducing piston pump. According to the detecting method, the reducing piston pump is used for vacuating each branch pipeline in a subsection manner, then whether the pipeline air pressure is stabilized around an approximate vacuum air pressure value is detected so that gas tightness of the oil-gas separation device is automatically analyzed and judged, a gas tightness trouble spot is positioned in the testing process, a method and an evidence are provided for automatically detecting the oil-gas separation device, and a large amount of production assembly time of a whole oil chromatography online monitoring system and the maintaining workload after field installation are saved. A worker can test whether the function of the oil-gas separation device is normal under the offline condition, and then the oil-gas separation device is mounted into an oil chromatography testing system and in the scene, so that unnecessary repeated labor is reduced, and the efficiency for finding trouble spots and eliminating the trouble spots is improved.