Graphitic carbon nitride (CN) is considered to be a promising metal-free photocatalyst, but the charge carrier separation and rapid complexation of electrons and holes are difficult to modulate for effective and comprehensive photodegradation of TC. In this work, a MoS₂ QD decorated sea-urchin carbon nitride nanohybrid was prepared via a water bath method. In comparison to the 85.6% degradation efficiency of sole sea-urchin carbon nitride (SUCN), the optimized 5% MoS₂ QDs/SUCN (5%MSUCN) demonstrated a removal efficiency of 96.0% towards aqueous tetracycline (TC) under visible light. Such an outstanding photoactivity was mainly attributed to the interfacial synergistic effect of MoS₂ QDs and SUCN, in which the outstanding charge extraction ability and quick surface kinetics of MoS₂ QDs were primarily accountable for the subsequent deep photodegradation behavior of 5%MSUCN. A series of factors that may affect photoactivity of 5%MSUCN include pH values, inorganic ions, and organic matter. Finally, the deep photodegradation mechanisms of TC and intermediates were proposed based on ex situ HPLC-MS. This work not only explains the deep decomposition of TC but also presents a facile approach for designing and manufacturing carbon nitride nanohybrids.