Abstract: To address the environmental persistence of neonicotinoid pesticides, boric acid (B) modified bismuth oxychloride (BiOCl) with enriched oxygen vacancies (VO), designated as B-BiOCl-VO, was successfully engineered through a facile solvothermal strategy. The strategic surface modification with B species modulates the electronic structure of BiOCl, effectively narrowing the optical band gap to 2.78 eV while boosting visible-light absorption. Experimental evaluations reveal that B-BiOCl-VO exhibits superior photocatalytic efficiency for imidacloprid (IMI) degradation, achieving complete removal within 100 min and 94.6% total organic carbon (TOC) removal within 3 h, with a reaction rate 1.29 times that of BiOCl-VO. Beyond its remarkable stability over five cycles, the synergistic mechanism elucidated by density functional theory (DFT) calculations demonstrates that the presence of surface B species significantly facilitates VO formation by reducing the formation energy from 3.27 to 0.52 eV. This surface functionalization concurrently strengthens the adsorption and activation of reactants, thereby accelerating the spatial separation of photoexcited charge carriers. These synchronized effects result in an enhanced production of primary oxidative species comprising superoxide (•O2⁻) and hydroxyl radicals (•OH). Furthermore, molecular-level DFT analysis combined with liquid chromatography–mass spectrometry (LC–MS) identification reveals that the nitroimine, imidazolidine, and chloropyridine-related regions of IMI serve as preferential reactive sites during reactive oxygen species (ROS) induced degradation. This research provides a robust theoretical and practical guideline for the atomic-level design of advanced bismuth-based materials for environmental remediation via strategic surface engineering.