Abstract: Based on analyzing the property of suction between loess grains and the micro-process of loess subsidence, the relation between the motive force or resistance for sliding of microstructural units with water content and soil pressure are discussed. A theory of synthetic effect of microstructure and generalized suction on the formation of loess subsidence is put forward. The relations between loess subsidence with water content, pressure or depth of soils are theoratically analyzed, and the inherent regularity of loess subsidence is open out. The motive force and resistance for structural reconstruction are caused by effects of gradual loss of generalized suction in small pores and gradual increase of absorption suction in larger pores after soil absorbing water. In this paper, it is thought that the process of subsidence is just a dynamic course of counteaction process between microstructre reconstructing motive force and resistance. Lastly, the basic conditions for subsidence and primary factors affecting subsidence are talked over. This study should be of theoretical significance in quantitative research of loess subsidence.