This paper proposes the view that the width of source fault plane got saturated in seismogenic layer when the energy reaches certain threshold value. Based on the view, combining actual seismic data,the author draws the conclusion that the lower-limit magnitude of strong earthquake is 7.1 in Chinese Continent,and the thickness of seismogenic layer is 20km. In the condition of the same medium strength, the relative expression between source linear dimension and magnitude is derived in the case of different fault plane width. The research suggests that the width and length of source fault plane of shallow focus shock in China is complementary. As a result, it is a unified expression among the area Of-source fault,dislocation, seismic moment with magnitude (M=5-8.5).