An improved failure wedge model is proposed to predict the ultimate lateral resistance of rigid piles on slopes. The emphasis of the improved failure wedge model is based on the three-dimensional combination forces and the hyperbolic p-y criterion, which considers the side friction force between the pile and the soil. A judgment method to determine the p-y curve based on theoretical analysis is proposed. To validate the improved failure wedge model, the method in this research is compared with the three-dimensional finite element (3D FEM) tests and existing solutions by using the hyperbolic p-y criterion and Finite Difference Method (FDM). The influence on the lateral pile response of rigid pile at different slope angle (p-y curves and the load–displacement curves), and the effect of the adhesion factor on the lateral rigid pile response are discussed by combining with numerical simulation and existing solutions, respectively. Through comparison, it is found that the curves predicted by the method proposed in this research are in good agreement with those obtained by FEM and actual examples. The results show that the improved failure wedge model is suitable for representing the pile-soil interaction for the lateral rigid pile on slope.