Abstract: Drilled piles and internal supporting structures are widely used at present especially in the foundation pit supporting works of municipal industry. The traditional uplift stability analysis method based on the the Prandtl formula for calculating the ultimate bearing capacity which, however, is not suitable for narrow foundation pits. Starting from the definition of the narrow foundation pit and according to the basic principle of the Prandtl formula for calculating the ultimate bearing capacity, the failure modes and relevant parameters suitable for the narrow foundation pit are reassumed, and the corresponding formula for calculating the ultimate bearing capacity of foundation is derived. Example shows that the proposed formula can better reflect the actual conditions of narrow foundation pits, and the safety factor is appropriately raised.