In order to achieve the construction of environmentally-friendly, efficient, and highly precise modular assembled steel structure buildings at construction sites, this study introduces two types of assembled square hollow column-truss beam connections: one with diagonal braces and one without diagonal braces. Four full-scale cross-shaped specimens were meticulously designed for the purpose of conducting cyclic loading tests. The seismic behavior of these specimens was thoroughly investigated under both beam failure and diagonal brace failure modes, and the underlying causes for the occurrence of these failure modes were elucidated. Furthermore, the impact of diagonal braces on crucial aspects such as ultimate bearing capacity, ductility, stiffness, and energy dissipation capacity was comprehensively evaluated. Finally, an analysis of the mechanisms underlying these two failure modes was conducted, and the design suggestions of joints with and without diagonal braces were respectively given in combination with the failure modes and mechanical performances. Remarkably, the experimental results indicate that the specimens without diagonal braces exhibited commendable seismic behaviors in terms of plastic deformation and energy dissipation capacity. The incorporation of diagonal braces effectively transferred plastic deformation from the truss beam to the end of the diagonal brace, resulting in a significant enhancement of the connections' bearing capacity and stiffness. However, it should be noted that the plastic deformation capacity of the truss beam was constrained prior to diagonal brace failure. During the whole loading process, the connection between the column seat and the truss beam of the four specimens did not show any damage phenomenon, and there was no obvious slip phenomenon of the flange plate between the column and the column seat, which indicates that the column seat type connection was reliable.