To investigate the bearing characteristics of pile foundations on clay slopes under horizontal loads and clarify the influence of a key factor, namely slope angle, on the performance of the pile foundations, model tests were conducted on horizontally loaded piles on clay slopes. The impact of slope angle on the horizontal displacement and bending moment of the pile foundations was explored, thereby revealing the horizontal bearing characteristics of the single pile under horizontal load on the slope. The numerical simulation method was used to analyze the influence of slope angle on the deformation mode and p-y curve of pile foundations. The results show that as the slope angle increases, the influence of the slope effect becomes gradually prominent, and the depth of the maximum bending moment point of the pile foundation has shifted upward. The horizontal displacement of the pile foundation increases. Compared with that in the flat site, when the horizontal load is 40 N, the horizontal displacement of the single pile on a 30° slope increases by 40%. Additionally, the initial stiffness and ultimate soil resistance of the p-y curve of pile foundations both decrease with the increase of the slope angle.