Photocatalytic nitrogen fixation, as a low-cost and promising technology, needs efforts to explore the photo catalyst with high activity and stability. In this study, the polyoxometalate (POM) cluster of [H4SiO40W12] (SiW12) has been successfully covalently combined with the KOH-modified graphitic carbon nitride nanosheets (K-C3N4) through the phosphate-bridged strategy. With POM acting as the co-catalyst, SiW12/K-C3N4 nano composites show excellent photocatalytic nitrogen fixation efficiency (353.2 mu M g(-1) h(-1)) in water under light irradiation. It can be ascribed to the enhanced separation efficiency of charge carriers through the formation of "phosphate-bridged" bonds between K-C3N4 and SiW12, and the increased oxidation capacity of water by bridging SiW12. Moreover, K doping and bridging SiW12 improves the adsorption and activation of nitrogen, which can be conductive to the reduction of nitrogen to ammonia. This work provides a feasible route to design and synthesize nanocomposite materials with exceptional performance for photocatalytic nitrogen fixation.