Pile foundation is easy to develop necking during construction and service, which will bring potential safety hazards to the superstructure. Clarifying the characteristic of horizontal bearing capacity for necking pile with cap is important to engineering design and reinforcement repair of pile foundation. Transparent soil made by fused silica sand and pore liquid with the same refractive index is utilized to carry out model experiment and investigate the effect of necking on horizontal bearing capacity of necking pile with cap. The soil deformation around piles were obtained after process the soil particle speckle field assisted with Mat-PIV incremental program. Subsequently, three-dimensional finite element numerical simulation is incorporated to explain the change reason of horizontal bearing capacity induced by necking. Results show that necking will weaken the transfer ability of horizontal load along pile shaft, and drive the deformation of upper pile shaft and soil anti-force on pile side to increase. The horizontal bearing capacity decreases with the increase of necking dimension, or decrease of necking position away from pile top. The pile bending can cause pile cap to develop oblique downward displacement, which will exert vertical pressure on the soil. Subsequently, the strength of the shallow soil is enhanced and the developing friction limit pile deformation, resulting in improving the bearing capacity of the pile.