The multimetallic sulfur-framework catalytic site of biological nitrogenases allows the efficient conversion of dinitrogen (N2) to ammonia (NH3) under ambient conditions. Inspired by biological nitrogenases, a bimetallic sulfide material (FeWSx@FeWO4) was synthesized as a highly efficient N2 reduction (NRR) catalyst by sulfur substitution of the surface of FeWO4 nanoparticles. Thus prepared FeWSx@FeWO4 catalysts exhibit a relatively high NH3 production rate of 30.2 ug h−1 mg−1cat and a Faraday efficiency of 16.4 % at −0.45 V versus a reversible hydrogen electrode in a flow cell; these results have been confirmed via purified 15N2-isotopic labeling experiments. In situ Raman spectra and hydrazine reduction kinetics analysis revealed that the reduction of undissociated hydrazine intermediates (M-NH2-NH2) on the surface of the bimetallic sulfide catalyst is the rate-determing step for the NRR process. Therefore, this work can provide guidance for elucidating the structure–activity relationship of NRR catalysts.