Two-dimensional single-atom catalysts (2D SACs) have attracted much more attention in the field of ammonia synthesis via electrochemical nitrogen reduction reaction (NRR). In this work, we comprehensively study the NRR performance of 28 transition metals SACs loaded on 2D MoSe2 using density functional theory calculations. In view of thermodynamics and electrochemical stability, adsorption of N2, selectivity and activity of NRR, Mn, Os, [email protected]2 exhibit outstanding performances with the most favorable potentials of -0.39 V, -0.36 V and -0.36 V, respectively. The excellent NRR performances are comprehensively related to the electronic states, kinetics, and NN bond length. Furthermore, aiming at three vital processes of NRR (adsorption and activation of N2, adsorption strength of intermediates, and the limiting potential), three independent descriptors (φ, ICOHP, ΔG*N) are adopted to make the activity trend clear. The results are of guiding significance to the design of high-efficiency NRR using SAC.