Abstract: The main span crossing the Weihe river in Yinchuan to Xi'an railway is a concrete continuous box beam with steel trusses, whose length is 4×60 m. For the problem of uncertain performance of a semi-rigid joint in design, the initial rotational stiffness of an external composite truss joint is studied. First, the joint zone was defined. Then, by the transmission path, the joint zone was simplified into three effective mechanical components:a chord, a PBL shear key group, and an exposed joint plate. Based on the principle of component method, the spring stiffness expressions of effective mechanical components were derived, and the initial rotation stiffness function expression of the joint was obtained. A finite element model of the joint was established. By comparing the numerical results and the theoretical calculation results, the validity of the model and the correctness of theoretical analysis were verified. The research results laid the foundation for the study of the moment rotation curve of the external joint.