A number of middle-scale triaxial tests are performed to investigate the wetting deformation behavior of a coarse-grained material from the Xiaolangdi declined-core earth and rockfill dam. The test results indicate that the wetting deformation of coarse-grained materials is influenced by confining pressure and wetting stress level. There is a hyperbolic relationship between the wetting axial strain and wetting stress level. The wetting axial strain is linearly related to confining pressure. Instead, the wetting volumetric strain is linearly related to wetting stress level as well as confining pressure. Based on the above deformation regularities and single-line method, a mathematical model is proposed for predicting the wetting deformation of coarse-grained materials. There are six parameters in this model and their definitions are clear, and the method to determine parameters through the indoor tests are given. The fitting curve of the wetting model is identical with the experimental curves. The fact that the properties of coarse-grained materials can be well reflected by the proposed model proves the practical validity of the model.