Electrocatalytic N-2 reduction is one of the most promising ways for green and sustainable production of NH3. However, a mechanistic understanding of the N-2 reduction process remains very limited. Herein, a surface-hydrogenation mechanism for the N-2 reduction reaction is proposed, which can well address the recently emerged sharp discrepancies between experiments and computations. Our results reveal that surface hydrogenation can drive N-2 reduction reaction on catalysts with weak N-2-binding strength (i.e., noble-metal catalysts) at low potentials. Instead of N-2 adsorption, the reduction of H+ is found to be the first step, which is also the potential determining step of the whole process. N-2 can be activated and reduced into *N2H2 subsequently by overcoming relatively high energy barriers, which determines the total reaction rate. Moreover, the cooperative effect of surface *H and the catalysts plays a key role in the activation of N-2. Our work not only provides new insights into the N-2 reduction reaction, but also paves a promising way for advancing sustainable NH3 production.