The nonlinear FEM models of concrete-filled square tubular column with internal diaphragms and steel beams with reduced beam section bolt-weld joints, involving geometric, materials and contact nonlinear, were established. The 3D models of beam-to-column joints with internal diaphragms by 3D solid elements are founded. The bearing capacity behavior of joints under the monotonic loading are analyzed, including P-Δ curves, the stresses distribution of beam, the dissipating energy ability and destruction form. The analysis results show that the initial stiffness of joint with reduced beam section is close to that of non-reduced beam section joint. The bearing capacities of joints with reduced beam section are lower a little than that of non-reduced beam section joint. The plastic hinge in the joint with reduced beam section is removed to the reduced beam region.