This paper proposed a novel modular steel structure system, which arranges the improved module units at interval and integrates them with the coupled unit, to avoid the structural redundancy resulting from the laminated-beams and group-columns.In view of the assembly characteristics of the novel structural system, this paper further proposed a beam-column joint, which connects the beams on both sides separately through fully-welded connection and fully-bolted connection. To investigate the seismic behavior of novel joint, quasi-static tests of three full-scale cruciform joint specimens were carried out, the failure modes, hysteretic curves, envelop curves, stiffness degradation and energy dissipation capacity of joints were analyzed, the effect law of vertical-splicing bolt connection type and column wall thickness in panel zone on the seismic behavior of joint was discussed. The results show that, seismic behavior on both sides of joint specimens exhibit some differences.The hysteresis curves of the coupled side joint are in bow shape and have good rotation behavior, the modular side joints exhibit shuttle shape hysteretic curves but their rotation ability are poor. Thickening the column wall in panel zone is an effective way to improve the seismic behavior of the joint.High strength bolts applied in vertical splicing could improve the hysteretic behavior, stiffness and energy dissipation capacity of the coupled side joint compared with through high-strength bolts.