Kailu sag is located in the southwest of Songliao basin with good oil and gas geological conditions, whereas its research degree is low. The study is mainly focused on the secondary structural units in the sag, the research is lacked on the overall structure and basement characteristics of the sag. In order to study the structural characteristics of the Kailu Sag and predict the favorable oil and gas areas, the paper is based on the gravity data and the previous research results such as geology and seismic data. After that, the paper identifies and infers the fault structure system of the study area combined with the actual geological background of the study area and previous relevant research results. To improve the accuracy of the basement depth by using gravity data of the basin, the basement depth was calculated by Parker’s method and the correlation analysis method, combined with reference to previous related research results. According to the interpretation results of gravity data and related data above, predicting reasonably the favorable oil and gas areas. The results show that the gravity characteristics of Kailu Sag are macroscopically the distribution of high and low gravity from northwest to southeast, which reflect the alternative distribution of protuberance and fault depression in the sag from northwest to southeast. Kailu Sag has complex faults and the boundary of the sag is mainly controlled by near SN, EW and two groups of NE faults. The Kailu sag can be further divided into 8 secondary structural units, which are bounded by fracture. The Cenozoic, Mesozoic Cretaceous and Jurassic developed in the sag. The petroleum geology condition in Ping’an fault terrace in Qinghe—Weiliansu fault depression zone in western sag, gentle slope belt in Manghan fault depression in eastern sag and Nanaimanqi fault depression in southern sag are outstanding, which make it a favorable hydrocarbon area.