A sanitary landfill with significant sliding indications was studied through field investigations and numerical analyses. A translational deformation mode was discovered, and the leachate level of the landfill reached approximately 0.8 times the height of the waste body, which was extremely adverse for stability. Subsequent analyses revealed that the sliding surface started from the top of the landfill slope, developed along the liner system, and ended at the toe of the slope. To improve the stability of the landfill slope, a vertical drainage system was constructed. The leachate of the drainage wells was extruded into the header pipe by high-pressure air so that the possible clogging of pumps could be prevented. The effect of the new drainage system was verified by the subsequent decrease in leachate level and horizontal displacement rate. This case reveals that the vertical drainage system is an effective control measure for unstable running landfills.