The electrochemical ammonia synthesis has attracted increasing attention due to its energy saving characteristics. However, developing novel electrocatalysts and their mechanism remain great challenges. Here, several transition metal (TM) atoms doped on phosphorene were studied as N2 fixation electrocatalysts by using density functional theory (DFT) calculations. The results demonstrate that single Ru atom doped phosphorene shows an excellent catalytic activity for ammonia synthesis via the enzymatic pattern. A small overpotential of 0.696 V is achieved for this process. The effect of oxidation in the catalyst was also discussed in our work. Oxidation deactivates the catalyst, which should be avoided in the experiment. Our outcomes offer a novel perspective for single-atom catalytic ammonia synthesis with phosphorene as a substrate.