Covalently bonded bridging between different semiconductors is a remarkable approach to improve the transfer of charge carriers at interfaces. In this study, we designed a ternary heterojunction (MBG) combining of molybdenum diselenide (MoSe2), black phosphorus nanosheets (Bpn) and graphitic carbon nitride (GCN). Among this MBG of MoSe2/Bpn/GCN, (i) the covalently bonded bridging effect between Bpn/GCN facilitates directional charge carrier transfer, meanwhile (ii) a Z-scheme heterojunction is formed between MoSe2/GCN to enhance the separation of photogenerated carriers. Furthermore, (iii) this composite exhibits an increased absorption for visible light. Using this MBG, photocatalytic degradation of over 98% of moxifloxacin is achieved within 20 min, with O2•− confirmed as the primary photocatalytic active species. These findings provide novel insights into the construction of efficient heterojunction by covalently bonded bridging.