Pile diameter optimization is an important part of piled raft foundation analysis for the purpose of minimizing the differential settlement. Based on a general analysis method of piled raft foundation, this paper proposed a pile diameter optimization model with the genetic algorithm. The optimization model contains an objective function of differential settlement and other nonlinear constraints. Detailed process steps are listed. Implementation of pile diameter optimization is illustrated by an example, with a contrast before and after the optimization of foundation settlement, pile load distribution, raft bearing ratio, shear and bending moment of the raft. The influences of raft thickness, piled foundation parameters and soil parameters on the optimal diameter are also studied by parameter analysis. Results show that piled length and soil property have a significant impact on radius optimization than pile body material properties and raft depth.