Nitrogen fixation is one of the most important issues but a long-standing challenge in chemistry. Here, we propose FeN3-embedded graphene as the catalyst for nitrogen fixation from first-principles calculations. Results show that in view of the chemical coordination, the FeN3 center is highly spin-polarized with a localized magnetic moment substantially to promote N-2 adsorption and activate its inert N-N triple bond. The synergy between the graphene and FeN3 equips the system with novel features for the catalytic conversion of the activated N-2 into NH3 via a six-proton and six-electron process, following three possible reaction pathways at room temperature. Our findings provide a rational paradigm for catalytic nitrogen fixation that would be conducive to ammonia production.