Bearing capacity of cast-steel joint with branches in a tree-like column structure is important when adopted, since failure of the joint will surely lead to the collapse of its whole superstructure. In this paper, by using the means of numerical simulation and experimental verification, mechanical behavior of three-branch cast-steel joints in the tree-like column structure under eccentric load was studied. A typical full-scale cast-steel joint with three branches was first tested under eccentric loads. Numerical analysis of the cast-steel joint with three branches under eccentric load was then carried out through ANSYS and SolidWorks. Failure mechanisms of this kind of joint were analyzed and the main failure mode was summarized. Finally, the formula for predicting load-carrying capacity of the cast-steel joint with branches under eccentric forces was proposed. The results showed that the failure mode of the joint under eccentric load is the buckling failure at the end of the compression side of the main pipe, and the formula based on the main failure mode is applicable in engineering and building designs.