Abstract: The soil microstructure is a kind of states of substance, which cou ld be described with a conceptual model possessing tree structure. Based on ima ge processing technology, a technical way to quantify the microstructure and a method to determine the indefinite structural factors are put forwards. By analysis of fractal geometry, the influence of natural structure and pressure on the subsidence of loess and the regularities of reaction between water an d soil have been studied. And then the mechanism for control on the subsidence due to fractal structure is suggested.