The method that investigating the relationships between macroscopic and grain-scale parameters of unsaturated soils is proposed， which set up the calculation models of discrete element in different porosity ratio and moisture content under different stress path. By the discrete element program， the existing PFC3D （particle flow code in three dimensions） was improved，and one-dimensional consolidation test of discrete element specimens in the different particles bond strength was carried out to determine the structure yield stress. In this paper， the relationship between intergranular bond strength and moisture content is established by means of structural yield stress， which can reflect the value of the actual sample size and the different water content and provide the idea to study the basic mechanical properties of unsaturated soil through PFC3D.