Abstract: The density and magnetic interfaces have been gained from the calculation based on gravity and magnetic data in Tianjin area. The result show that the interface, whose average depth is about 6.5 km deep under earth’s surface, is coincident with the relief of Cenozoic era surface in this area. So the viewpoint is taken shape that the strata have been formed steady and continuously since beginning of the Cenozonic era; There has been the activity of igneous rock obviously in south by west of the area. Combining the analysis and study on magnetic interface ,the authors think that the crust structure here is controlled by “a pile up slopes poker face to face”, which is a accepted as the growth model of Bohai gulf basin widely; There is a deepmajor fault along northeast in west of the area. Its extension in deep is to southeastward which coincides in most of the faults in this area. More attention should pay to the activity of this fault.

Key words: gravity and magnetic anomaly, interface study, deepmajor fault, deep tectonics