<p>Horizontal resistance can vary significantly for gravel soil slope sites with different gradients. In this article, the impact of slope gradient on the horizontal resistance of soil is investigated with three-dimensional physical simulations, and the data of the displacement of the pile top and soil pressure for four piles under various gradients are analyzed. These data reveal that the distribution of horizontal resistance of slope site soil is related to depth and displacement, based on a relationship between pile-side soil resistance values for sites of various gradients and values for horizontal sites. A gradient factor, which is only related to the slope gradient, is used to characterize the impact of gradients on the soil resistance modulus and ultimate resistance. The research results have developed the theory of foundation design for building structures, promoting the reliability evaluation of pile soil systems towards a more scientific and rigorous direction.</p>