On the basis of structural analysis of 3D seismic data in Nanpu sag, applying faulting model with pre-existing weaknesses, combining with sand modeling results, the fault system and structural styles in Nanpu sag were systematically analyzed. The results show that, the structural style is typical oblique extension. The complicated “Y” shaped style , which is jointly controlled by listric (or ramp-flat) boundary normal faults and pre-existing faults, is the basic structural style in Nanpu sag, followed by complicated “X” shaped, “domino”, “Ladder” and “imbricate faulted blocks” structural styles. 4 fault systems are divided in the plane, that are Beipu-Laoyemiao fault system, Gaoliu fault system, Baigezhuang fault system and Nanpu fault system. Among them, Gaoliu fault system and Nanpu fault system can be further divided into 2 and 3 sub-fault systems respectively. While, it can be divided into two fault systems vertically: lower fault system (E2s3 tectonic layer) and upper fault system (E2s1-Q layers). There are four kinds of fault assemblages: “parallel-like”, “parallel intertwined” “comb-like” and “broom-like”. Cenozoic structural deformation of Nanpu sag can be reasonably explained with two-phases extension model: structural deformation of E2s3 era is caused by NW-SE direction extension with Mesozoic pre-existing weaknesses, while structural deformation since E2s1 is caused by north-south direction extension with pre-existing faults formed in E2s3 era, and E2s2 era is stress transition phase. There are significant segmentations in the sag boundary faults (Xinanzhuang Fault, Baigezhuang Fault and Gaoliu Fault), the strike, properties, activation of different segments showed a significant difference, and significant changes occurred in the different stages of the tectonic evolution. The tectonic framework by the sag boundary faults, Nanpu Fault and Gatou Fault in sag played an important controlling role on the fault formation and evolution of Nanpu sag. The complicated pre-existing faults were the root causes of lateral variation of fault system.