This work proposes a square concrete-filled steel tubular (CFST) joint system, in which vertically placed diagonal plates help transfer the tensile load from the beam flanges. Then, a variable density method is used to validate that the exterior-diaphragm of square CFST connections can be largely reduced or even eliminated. Seven square CFST connections with diagonal plate were experimentally and comparatively investigated under tension. Studied parameters included the width and thickness of beam flange, length and thickness of diagonal plate. The failure mode, initial stiffness, yield load, and peak load were analyzed based on the test results. The failure modes of the tested specimens included fracture of the beam flanges, fracture of welds—between the beam flange and the steel tube corner—after the beam yielding, and fracture of welds at the corner of the steel tube column and fracture of the diagonal plate. The failure mode of the connections depends on the strength of the beam and connection, and varying thickness and width of beam flanges would change the strength of the beam, which affects the failure mode. Finally, theoretical formulas are presented to predict the yield capacities of the connections, and the predicted results agree well with the test results. The diagonal-plate connection has the advantages of conveniently concrete pouring and not occupying indoor space, and thus provides an alternative for CFST construction.