Abstract: Qianbei sub?sag is the biggest one in Changling fault depression in southern Songliao basin, with low degree of natural gas exploration. This paper analyses the origin of deep gas in this sub?sag using natural gas component, isotope and lighter hydrocarbon data, combined with the characteristic of hydrocarbon source rocks. The result shows that the deep source rocks are dominated by humus kerogen, characterized by high abundance of organic matter, and good source rocks of Type?Ⅱ1 and Type?Ⅱ2 are distributed in Yingcheng formation, with post?maturity evolution degree; methane accounts for absolute advantage of natural gas component, nohydrocarbon gas contentis low; ethane isotope value is somewhat lighter, methane and ethane have obvious carbon isotope reversal phenomenon, corresponding to different types of kerogens: the methane is partial humic type and the ethane partial sapropelic type