The concept of improving in-situ nitrate removal was demonstrated in a CdS nanoparticles (NPs)-assisted periphyton bioelectrochemical system (PCdS-BES). Compared to the control (periphyton bioelectrochemical system, P-BES), nitrate reduction to nitrogen gas by the PCdS-BES was enhanced by 1.5 times on day 7 under stimulated sunlight irradiation (20Wm(-2)), avoiding nitrous oxide emission. The presence of CdS NPs optimized the community structure of periphyton, enhanced its activities (represented by ATPase), stimulated more extracellular polymeric substance (EPS) production and increased the relative abundance of electroactive bacteria strains (e.g. Family Xanthomonadaceae, Hyphomonadaceae and Sphingobacteriales). The enhancement of nitrate reduction under irradiation was primarily attributed to the synergistic effect of EPS, electroactive bacteria strains and CdS NPs. Specifically, CdS NPs provided photoexcited electrons under light irradiation. The EPS facilitated the stability of CdS NPs in the periphyton matrix and separation of photo-induced electron-hole on the surface of CdS NPs. EPS served as extracellular electron transfer mediators for electron transfer from CdS NPs to microorganisms. The electroactive bacteria were beneficial to the acquisition of electrons produced by CdS NPs under irradiation, promoting catalytic nitrate reduction. This study gives an insight into the mechanism of nitrate reduction via the synergistic action of photoexcited electrons, EPS and electroactive bacteria. The successful combination of photocatalyst (i.e. CdS NP) and microbial community in BES also provides a promising approach for nitrate removal.