Despite that the stability of landfill slope has been receiving increasing concern from all fields, the spacing of stabilizing piles in the landfill slope has not been widely researched, not to mention the influence of groundwater on pile spacing. In view of this, we investigated the maximum spacing of stabilizing piles in landfill slope in two conditions (in the presence and in the absence of groundwater) in the light of the unified strength theory. Firstly, we employed GEO-STUDIO to searching for the potential sliding surfaces of landfill slope in the aforementioned two conditions, acquired the thrust distribution behind the stabilizing piles, and built a soil arch model between piles. Subsequently, we deduced the calculation formula of maximum spacing of piles based on the strength condition and equilibrium condition in the unified strength theory. Finally, we determined the maximum spacing of piles on the basis of studying the interaction among sliding bodies in the multilayer landfill slope. The calculation results illustrated that in the presence of groundwater, the volume of the sliding body and the residual thrust of the slope are larger than those in the absence of groundwater; but the stability coefficient and the maximum pile spacing are smaller. In addition, we applied the calculation formula proposed in this paper to a landfill slope in Shenzhen, and results demonstrated that the formula could serve as reference for determining the maximum pile spacing in multilayer landfill slope.