The current standards have not yet specified the formula for calculating the bearing capacity of Π-shaped welded square tube joints. Based on experimental validation of finite element analysis accuracy, this paper utilizes Abaqus software to numerically simulate 85 sets of joints under axial load, obtaining the ultimate bearing capacity of Π-shaped joints. Subsequently, parameter analysis and regression analysis were conducted to identify factors influencing the ultimate bearing capacity of Π-shaped joints and the modified calculation formula. The results show that the width ratio of branch pipe to main pipe β has a great influence on the ultimate bearing capacity and initial stiffness of the joint. Increasing the width of branch pipe can significantly improve the ultimate bearing capacity of the joint. The load imposed on the branch pipe is jointly borne by the bending and shear effects on the upper surface of the main pipe and the side wall of the main pipe. The failure mode of the joint also depends on β. The larger the width thickness ratio of the main pipe 2 γ, it means that the connection area between the upper flange of the main pipe and the branch pipe becomes more slender, which reduces the bending stiffness of the upper flange of the main pipe, and therefore reduces the bearing capacity and initial stiffness of the joint. The branch pipe to main pipe height width ratio η and the branch pipe spacing have a certain impact on the ultimate bearing capacity of the joint. Increasing the height of the branch pipe section and the branch pipe clearance, that is, increasing the intersection area of the branch pipe and the main pipe along the longitudinal direction of the main pipe, makes the branch pipe transfer load in the wider area of the main pipe flange. The plastic area of the node is larger and the material is more fully utilized, thus improving the carrying capacity of the node. The thickness ratio of branch pipe to main pipe τ has little effect on the ultimate bearing capacity and initial stiffness of the joint. The increase of the wall thickness of the branch pipe improves the bearing capacity of the branch pipe, but the ultimate failure of the joint is the yield failure of the upper flange of the main pipe rather than the failure of the branch pipe. Therefore, the change of the thickness of the branch pipe has no obvious effect on the bearing capacity of the joint. Based on the analysis results of the finite element model, a parameter equation for calculating the ultimate bearing capacity of welded square tube Π-shaped joints was proposed through curve fitting, and the accuracy of this equation was evaluated, providing a reference for further research and engineering applications of such joints.