Massive voids form in underground mines due to the continuous removal of valuable minerals. Backfill and pillars are commonly built into the voids to provide both local and regional support. The backfill material cannot rigidly support the upper load and acts as a secondary support system to the pillars. Confinement effect effectively enhances the strength of the pillar; however, the lateral stress mobilized within the fill is contingent upon deformation of the roof and pillars, which act as a passive support. Without large-scale volumetric compression, the backfill has no effect on the prepeak stage of the pillar. In this case, the friction resistance of the interface between the pillar and backfill plays a key role in increasing the load capacity of the support system. This paper presents an analytical solution based on the load transfer of friction force in the interface, which involves the shear behavior of the interface, the mechanical properties of the pillar, and the pillar dimensions. Experimental results showed that the bearing capacity of the support system was strongly dependent on the interface shear parameters, lateral stress, and fill ratio. Comparisons were made between the proposed solution and experimental results. The results may represent a workable understanding of the interaction mechanism between backfill and pillars and assist in the design and optimization of underground mine support systems.