Large diameter steel pipe rod generally adopts straight pile foundation or step foundation, and its economy is poor. When variable cross-section pile foundation is adopted, the large diameter part can be reinforced according to the pile cap and pile. The ABAQUS software is used to establish the refined finite element model of variable cross-section pile and straight pile respectively under the flat ground condition. The stress and displacement are controlled, and the response analysis under the same control condition is carried out. The variable cross-section pile and straight pile are compared. The results show that: 1) Under the conditions of downward pressure and upward uplift, the maximum displacement of the variable cross-section pile foundation is not much different from that of the straight pile foundation, and the maximum stress meets the strength requirements, but the variable cross-section pile foundation is more material-saving, so it is more economical to choose the variable cross-section pile; 2) Under the downward pressure condition, the maximum stress and maximum displacement of the variable cross-section pile foundation designed according to the pile cap are not much different from those of the variable cross-section pile foundation designed according to the pile, but the variable cross-section pile foundation designed according to the pile has better mechanical performance of the steel bar. Under the uplift condition, the vertical displacement of the variable cross-section pile foundation designed according to the pile is smaller, and the mechanical performance of the steel bar is better. Therefore, the variable cross-section pile foundation can be designed according to the pile.