According to the mechanism of pile-soil interaction under vertical load, a simplified calculation model for foundation pile settlement was established considering the nonlinear characteristics of soil stress and strain on the pile side. The theoretical solution of foundation pile settlement calculation was derived. Taking the foundation pile in ZX49R pier of Xiangfu road viaduct as an engineering example, the calculated settlement value of foundation pile using the load transfer method was verified by measured value, and the influence of pile diameter and pile length on foundation pile settlement was discussed. The results show that the settlement of foundation pile decreases with the increase of pile diameter and pile length, and increases with the increase of vertical load on pile top. The vertical settlement under load can be significantly reduced by increasing the pile diameter and pile length. However, when certain point is reached, the vertical settlement is impacted slightly by pile diameter and length. This study is considered to provide reference for the design of pile foundation engineering.