A strong earthquake (M 7.3) occurred in south Taiwan Strait on Sept.16,1994. In this paper,the its structure environment is analysed based on the tectonic unit, active fault, geophysical field, submarine morphology etc. The studies considered that the macroseism is situated at change location from continental crust to transitional crust. The seismicity in history is distributed along modification belt of deep-seated structure. The geological structure,hypocentral depth, type of earthquake, the characters of seismic wave in this region are different from middle and northern Taiwan strait. It has characteristics of the South China Sea system. The spatial pattern of seismicity before the earthquake revealed that the macroseism was result of fault activity along the north margin of the South China Sea system.