Abstract: The term "Petroleum System" has been widely discussed by petroleum geologists and has different definitions or interpretations. Maggon (1987) first identified a petroleum system and put emphasis on the genetic relationship between active source rock and the relevant petroleum accumulations. Petroleum System analysis is to comprehensively study the time-space arrangement and evolution pattern of source rock, reservoir rock, cap rock, migration, accumulation and preservation of hydrocarbon in a petrolifer- ous basin on base of basin analysis. A petroleum system includes an active source depression, relevant oil and gas accumulations or SG&O and basic elements necessary to form an accumulation and its process. This paper briefly introduces the study history, definition. characteristics and classification of petroleum system.