Pile foundation is easy to develop necking during construction and service, which would bring potential safety hazards to the superstructure. Clarifying the characteristics of horizontal bearing capacity of a necking pile with cap is important for engineering design and reinforcement repair. Transparent soil made by fused silica sand and pore liquid with the same refractive index is utilized to carry out model experiments and investigate the effect of necking on the horizontal bearing capacity of a necking pile with cap. The soil deformation around piles was obtained after processing the soil particle speckle field assisted with Mat-PIV incremental program. Subsequently, three-dimensional finite element numerical simulations are incorporated to explain the change reason of horizontal bearing capacity induced by necking. Results show that necking would weaken the transfer capacity of horizontal load along pile shaft, and drive the deformation of upper pile shaft and soil resistance on pile side to increase. The ultimate horizontal bearing capacity decreases with the increase of necking dimension or the reduction of the distance from necking to pile top. The pile bending can cause the pile cap to develop oblique downward displacement, which would exert vertical pressure on the soil. Subsequently, the strength of the shallow soil is enhanced and the developed friction limits pile deformation, resulting in the improved bearing capacity of the pile.