[Objective] The use of modified hydrophilic polyurethane resin binder (W-OH) solution at different concentrations to reduce leakage from gravel slag slopes was investigated to provide theoretical support for reducing leakages from coarse gravel slag and to improve vegetation restoration in rocky desertification areas. [Methods] A highway slag slope in the rocky desertification area of the Guangxi Zhuang Autonomous Region was investigated. Artificial erosion experiments were conducted on a sloping gravel surface and the treatments were different concentrations of added W-OH solution with different erosion flow rates. [Results] The slope that was not treated with the W-OH solution produced large amounts of leakage water and runoff on the slope only appeared when the scouring discharge was greater than 30 L/min. When the scouring discharge was 30 L/min, the runoff rate accounted for 18.3% of the incoming flow rate and 81.7% of the water was lost due to slope leakage. However, the average runoff and average infiltration rates for the slope after treatment with the W-OH solution increased as the inflow rate and W-OH solution concentration rose. The stable runoff rate for the slope treated with W-OH solution concentrations of 3%—6% increased by 200%—385.5% compared to that of the untreated slopes and the higher the concentration of the W-OH solution, the better the treatment effect on the slope. The infiltration runoff ratio decreased as the inflow rate and the W-OH solution concentration increased. [Conclusion] Treatment with the W-OH solution effectively reduced leakage from coarse-grained slag slopes and improved the water retention capacity of the slope. These improvements contributed to slope vegetation restoration.