Different types of unsaturated soils are widespread in nature. Due to the differences in grain size and mineral composition, the shear strength characteristics of the unsaturated soils are significantly different over a wide suction range. In order to establish a unified model for predicting the shear strength of different soil types over a wide suction range, a systematic investigation was carried out to examine the effect of soil type on the shear strength of unsaturated soils. An improved shear strength model is proposed based on the advantages of two traditional equations. The proposed model is verified by comparing the published experimental data and the predicted results obtained from the proposed and the existing equations. The conclusions are drawn as follows: (1) the shear strength for different types of soils over a wide suction range can be roughly classified into two types: peak type and steady growth type; (2) the proposed model demonstrates a good agreement between the predicted and measured shear strength results over a wide suction range; and (3) the physical meaning and the recommended values of the model parameters for different soil types are given.