The constitutive relationship between suction and degree of saturation is of great significance for estimating the shear strength and deformation behavior of unsaturated soils. A unimodal/bimodal soil-water retention curve (SWRC) is proposed considering the effects of various pore structures and the effects of void ratio on capillarity and adsorption. The different mechanisms of water retention through capillarity and adsorption are explicitly distinguished in the proposed model. The relationship between the capillary degree of saturation and suction is described as a specific function related to the characteristics of pore-size distribution, while the adsorptive degree of saturation is modeled considering the effect of capillary condensation explicitly. Subsequently, the decoupling formula of capillary and adsorptive saturation is further put forward. The formula lays a foundation for the model to account for the dependence of capillary part of SWRC on void ratio, which is consistent with the results from micro-scale tests. Eventually, an approach to estimate the shear strength of unsaturated soils with different initial void ratios has been proposed based on the improved SWRC model. The model is verified using data from water retention and direct shear tests reported for various types of soils in the literature.