Abstract: In order to improve its reinforcement effects, the bearing mechanism of a composite foundation with plain concrete piles reinforced by border piles under large loads and rapid loading is studied. The research shows that the composite foundation with plain concrete piles transfers most of the loads to the underlying layer at the pile end, and relatively reduces the compression and shear deformation of the shallow soft soil, and makes use of the compression and shear resistance of the deep soil. When the accumulated test loads reach 350 kPa within 42 days, the overall stability of the foundation is good. Under the large loads, the plain concrete piles in the composite foundation will incline or bend, and there is a hidden danger of tensile crack damage and damage of the pile body concrete at the junction of soft and hard soil layers. The damage of the pile body concrete shall be considered in the foundation stability analysis. The reinforced border piles have a significant stability control effect on the composite foundation with plain concrete piles, while the strength of the border piles is the same as that of the others under the same layout, and in this case the foundation cannot maintain stability. The stability control effects are mainly to limit the horizontal displacement of the shallow soil. The effective depth of the reinforced piles mainly depends on the resistance provided by the bearing layer. When the foundation with border piles and the others which have the same strength is fails, the damage of plain concrete pile body is orderly. The position of border piles belongs to that of early damage. The scheme of reinforced border piles is reasonable.