Abstract: Accurate determination of the effective stress coefficient is an important thing in the study of the effective stress of unsaturated soils. However, the existing effective stress coefficients have not well considered the effect of the microscopic occurrence of pore water in unsaturated soils on the effective stress. For this reason, the microscopic occurrence of pore water is analyzed firstly. It is recognized that the pore water in unsaturated soils can be divided into the contractile skin, adsorbed water and capillary water, and an extended three-phase porous medium model of unsaturated silt is established, considering the pore air, capillary water, and generalized soil skeleton. Based on the established model, an effective stress equation for unsaturated silt is derived by using an independent phase balance approach. The equation reasonably considers the influences of the capillary water on the effective stress by adopting a coefficient (i.e., the effective saturation of capillary water). In light of the derived effective stress equation, a shear strength equation for unsaturated silt is proposed. Finally, the proposed shear strength equation is verified by the experimental data on a reconstituted unsaturated silty clay under five different net confining pressures. The results show that compared with the existing two shear strength equations, the proposed equation better predicts the variation of the shear strength of the reconstituted unsaturated silty clay with matric suction. Therefore, under the unsaturated conditions, using the proposed shear strength formula to evaluate the stability of slopes, the bearing capacity of foundation, and the stability of retaining wall will achieve better results.