It is essential to explore high efficient catalysts for nitrogen reduction in ammonia production.Based on the first-principles calculation,we find that B/g-C2N can serve as high performance photocatalyst in N2 fixation,where single boron atom is anchored on the g-C2N to form B/g-C2N.With the introduction of B atom to g-C2N,the energy gap reduces from 2.45 eV to 1.21 eV and shows strong absorption in the visible light region.In addition,N2 can be efficiently reduced on B/g-C2N through the enzymatic mechanism with low onset potential of 0.07 V and rate-determining barrier of 0.50 eV.The “acceptance-donation” interaction between B/g-C2N and N2 plays a key role to active N2,and the BN2 moiety of B/g-C2N acts as active and transportation center.The activity originates from the strong interaction between 1π1π* orbitals of N2 and molecular orbitals of B/g-C2N,the ionization of 1π orbital and the filling of 1π* orbital can increase the N≡N bond length greatly,making the activation of N2.Overall,this work demonstrates that B/g-CaN is a promising photocatalyst for N2 fixation.