This study aimed to solve the problem of drilling shaft support under deep water-rich and weakly cemented stratum conditions. Thus, ultra-high-performance reactive powder concrete (RPC) is proposed as the wall construction material for the drilling shaft lining. The mechanical behavior, permeability, and microscopic characteristics of RPC were tested and analyzed. First, RPC samples were prepared by standard and autoclave curing with strength grades of C110 and C140, respectively, and their quasi-static mechanical properties were tested and compared. Stress-seepage coupling tests were then conducted under different confining and osmotic pressures to analyze the mechanical deformation, permeability evolution, and strength and deformation characteristics. The results showed that autoclave curing significantly improved the strength grade and early strength growth of RPC. Additionally, RPC generally exhibited low permeability characteristics while its permeability evolution law corresponded to the stress–strain relationship under triaxial loading. The permeability was observed to undergo three stages: a gradual decrease, steady development, and rapid increase. Finally, mercury intrusion porosimetry, X-ray diffraction, and scanning electron microscope microscopy analyses were used to determine the mechanism of improving macro mechanical properties and impermeability of RPC by the autoclave curing method based on pore structure characteristics and microstructure formation aspects.