3D plasmonic Bi metal@defective BiOCl hierarchical microspheres were constructed by a one-step solvothermal method. The effects of solvent thermal temperature on the microstructure and the photocatalytic performance were investigated. The photocatalyst prepared at 200 degrees C (Bi/BiOCl-200) showed most efficient visible photo catalytic activity for NOx removal. The enhanced activity can be ascribed to synergistic effects of oxygen vacancies and Bi metals. The oxygen vacancies in BiOCl induce the formation of an intermediate level to allow electrons transition from the valence band to intermediate level and then to the conduction band as revealed by density functional theory (DFT). The surface plasmon resonance (SPR) effect of elemental Bi enables the improvement of the visible light absorption efficiency and the promotion of the charge carrier separation as evidenced from the charge difference distribution between Bi atoms and Bi-O layers in BiOCl. The NO adsorption and reaction processes on Bi/BiOCl-200 were dynamically monitored by in situ infrared spectroscopy (FTIR). The mechanism of Bi metals and oxygen vacancies co-mediated photocatalytic performance on Bi/BiOCl was proposed based on the results of intermediate products analysis, radicals trapping and DFT calculation. The present work could provide new insights into the mechanistic understanding of the non-noble metal Bi-based plasmonic photocatalysts and offer a new technique to reveal the mechanism of gas-phase photocatalytic reaction.