Tree roots play a crucial role in hillslope stability, but quantifying their reinforcement remains challenging. This study aims to quantify the root reinforcement provided by Cunninghamia lanceolata across varying slope gradients based on in-situ pullout experiments. A total of 120 soil profiles were excavated to map root distribution across four slope gradients. Subsequently, 304 in-situ pullout experiments were conducted encompassing root diameters ranging from 1 to 8 mm. The Root Bundle Model Weibull was calibrated and coupled with root distribution data to quantify reinforcement contributions from a single tree to stands. It was found slope gradient significantly influences root distribution, with steeper slopes harboring coarser and more widely distributed roots. In-situ experiments revealed substantial variability in pullout stiffness and peak displacement for roots of the same diameter, with thicker roots exhibiting higher stiffness and greater displacement. Calculations indicate that root reinforcement exhibits an exponential decline with increasing distance from the stem but shows a marked positive association with slope gradient due to the influence on root distribution. Statistical analysis reveals that the area experiencing root reinforcement exceeding 10 kPa on a 40 degrees slope is roughly double that of 0 degrees and 20 degrees stands.