A new type of diaphragm connection between U-shaped steel reinforced concrete composite beams and concrete-filled steel tubular columns is introduced. An experimental study is conducted under low-cyclic repeated loading, and a finite element (FE) analysis of the test is performed using ABAQUS. The failure location, stress distribution, and a hysteresis curve of the joints obtained by finite element simulation are basically consistent with the experimental results. Based on the FE method, the effects of the wall thicknesses of the U-shaped steel beams, steel tube columns, and through partition on the seismic behavior of joints are investigated. Combined with test failures, the deficiencies of the joint structure are improved. Results showed that the wall thicknesses of the U-shaped steel beams had a significant effect on the seismic performance of the joints, whereas those of the steel tube columns and that of the through diaphragm had little impact on seismic performance. However, compared with the single increase in material size, reasonable construction measures could improve the seismic performance of the joints to a greater extent, take advantage of the superiority of composite materials, improve the efficiency of material utilization, and reduce engineering costs.