The catalytic mechanism is an important issue in catalytic reaction process. In order to enhance the photocatalytic efficiency of NO removal, we fabricate the Bi/BiOI nanos-phere photocatalyst via the in situ reduction of Bi³⁺ on the BiOI surface. Through a series of characterization analysis, we found the microstructure and optical properties of the catalyst changed significantly after the metal Bi was deposited on the surface of BiOI. Compared with pure BiOI, the prepared Bi/BiOI catalyst exhibited better photocatalytic activity for the removal of gaseous NO. Highly efficient visible light absorption and electron-hole pair separation are induced by surface plasmon resonance (SPR) of metal Bi. Meanwhile, by applying the in situ Diffuse Reflection Fourier Transform Infrared Spectrum (DRIFTS) and ESR spectra dynamically monitor the reaction process of Bi/BiOI photocatalytic oxidation of NO. Combining with the results of free radical capture, the reaction products were revealed from the molecular level. The mechanism of photocatalytic oxidation of NO by Bi/BiOI was proposed based on surface plasmon resonance (SPR).