Abstract: Low-reversed loading tests and fracture mechanism analysis were performed on the joints connecting a steel box column and H-section steel beam with through diaphragms, testing four joints with double linear enlarged junctures and one conventional joint, based on an elliptical fracture model coupled with an elliptical yield model taking into account welding defects and residual stress for constructional steel. The effects of double linear enlarged junctures on failure mode, ductility, load bearing capacity, rigidity degradation, and energy dissipation of the joints connecting a steel box column and H-section steel beam with through diaphragms were comparative studied. The evolution law of fracture and yield index near the joint panel zone were developed. The results of tests and numerical analysis show that the conventional joint undergoes brittle fracture at the edges of butt weld between beam flange and through diaphragm, where the geometry is discontinuous, while the plastic rotation is roughly 0.02rad, which does not meet the minimum requirement (0.03rad) in FEMA. For the joints with double linear enlarged junctures, the plastic hinge forms at the linear enlarged section of diaphragm, the plastic rotation reaches 0.03rad, and the load bearing capacity and the energy dissipation capacity increase by 10.1%~27.7% and 65.5%~123.6%, respectively, compared with those of the conventional joints. Drastic changes in geometry and stress concentration in the connection zone are mitigated by the use of double linear enlarged junctures, and the overlap and interaction of welds are avoided by moving the butt weld between beam flange and diaphragm to a section far away from the panel zone.