This paper analyzes the microstructure of 16 groups of soil sample from three different sites.Pictures of microstructure were processed with imaging processing program to acquire the geometric characteristic parameter and fractal dimension of pores.The range of pores′ fractal dimension is 1.816-1.936.The pore distribution function of unsaturated soil was established based on fractal geometry.The moisture distribution of loess under natural moisture was analyzed.The relationships of collapsibility coefficient with fractal dimension and porosity of unsaturated pores were analyzed with the method of regression analysis.A larger fractal dimension represents a more complex pore-structure.The collapsibility coefficient of loess increases with the increase of fractal dimension and porosity of unsaturated pores.Unsaturated pore is the main cause to form collapsibility of loess.