Abstract: Because of complicated development conditions of all level of faults and big difficulty in tectonic evolution research,there had no commercial quantitative petroleum found in northern sag of northern depression in the south of Yellow Sea basin up to now.Based on fine structure interpretation of 3 D seismic data of the northern sag,a research on both tectonic characteristics and trap styles in the northern sag was carried out,which is a prospect zone for petroleum exploration.The results indicate the faults evolution in this area can be divided into 4 stages,namely,the early stage,the middle stage,the late stage and the stage of sustained developing.There are distinct structure differences between the upper strata and the middle-down strata,the shape of middle-down strata varies intensely but the upper strata change gently.The main tectonic style of northern sag included graben,horst,fault terrace,flower structure and flower-liking structure.The north sag was controlled by the regional extension and strike-slipping,and in terms of trap styles,fault noses and fault blocks are the main styles developed in this place,and then tectonic-lithologic trap,which blocked by faults.Generally,the planar feature of trap styles in this area was characterized by zone.