In this paper, the shear strength formula of unsaturated soil proposed by Vanapalli et al. (1996) is demonstrated, and the applicability of the formula is validated by the test data published in literatures. Based on the definition of soil skeleton stress, the effect of pore fluid pressure and the other external forces on soil skeleton are considered separately, and the stress induced by all the other external forces excluding pore fluid pressure is defined as the effective stress. The shear strength of soil skeleton (the strength of soil) is due to the external forces induced skeleton stress and pore fluid pressure with different impacts. When the impact of pore fluid pressure is negligible, the skeleton stress induced by the other external forces determines the strength of soils. This paper illustrates why the effect of pore fluid pressure and the other external forces excluding pore fluid pressure can be considered separately to define the soil skeleton stress, then derives the relationship between the soil skeleton stress due to external forces, the total stress and pore fluid pressure. By discussing the contribution of the skeleton stress induced by external forces and pore fluid pressure on the strength, it is derived the shear strength formula for unsaturated soils. Moreover, the shear strength formula expressed by the external forces induced skeleton stress can be obtained. For saturated soils, the soil skeleton stress due to external forces is actually Terzaghi’s effective stress. For unsaturated soils, it is proved that the soil skeleton stress due to external forces determines the strength of soils using the test data available.