In order to solve the problem of soil resistance reduction caused by the existence of slopes, this paper carried out a comparison test of the horizontal loading model on the foundation piles in the flat soil and different slope conditions of the clay soil. The test results show that the load deformation curve and internal force variation of the laterally loaded piles under two conditions are basically the same, but the bearing capacity of the piles located on the slope body is obviously reduced. However, the reduction effect is closely related to the slope angle and pile location in the slope. The influencing factors of the soil resistance reduction effect are further quantitatively analyzed, and the quantitative relationship between the reduction effect and the position and slope angle of the pile in the slope is proposed. On this basis, the p-y curve formula of lateral loaded piles that can consider the soil resistance reduction effect at the slope is established by reasonably correcting the ultimate soil resistance and initial stiffness of the slope. The comparison between the theoretically calculated value and the existing experimental results shows that these two agree well with each other.