Abstract: The critical state of soil shows the end of its deformation process, which is very important for analysis and establishment of constitutive model for soil. The unsaturated silty sand is used in this study. The triaxial testing system (GDS) is used to study the critical state of soil with different dry densities, net stresses and suctions, and the variations of soil strength, volume change, water content and saturation during the process are investigated. The results show that in the process of shearing to critical state, the strength and deformation characteristics of unsaturated silty sand with different dry densities are different. With the increase of axial strain, the curve of deviatoric stress, strain and saturation of silty sand specimens with different initial dry densities tend to a stable value, which indicate the specimens are sheared to critical state. Based on the test results, linear critical state equation in the plane of q-, ν- and ν_w- are established. The critical state of unsaturated soil are finally discussed based on the variation of strength, deformation and pore water.