Electrocatalytic reduction of nitrogen is a simple, green and sustainable method for the production of NH3, so it is very necessary to find an efficient and low-cost electrocatalyst. Here, we systematically studied the catalytic performance of two 3d transition metal (TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) atoms loaded on C2N monolayer. Through density functional theory calculations and systematic theoretical screening, it is known that Sc and Ti loaded structures can undergo nitrogen reduction reactions (NRR). Unfortunately, the limiting potential (UL) of NRR is not very friendly and is not conducive to the production of NH3. In order to reduce the UL of NRR, we further studied the structure of B and Ti co-doping (B-Ti2 @C2N). As expected, the UL of the B-Ti2 @C2N via the consecutive pathway is 0.10 eV, with high electrocatalytic activity, which effectively inhibiting the competition of the hydrogen evolution reaction. This theoretical research not only adds a new member to the family of electrocatalysts for nitrogen fixation, but also further increases our physical insights on adjusting catalytic activity, which is expected to guide the rational design of improving the catalyst activity of NRR.