The electrocatalytic nitrogen reduction reaction (NRR), as an alternative green technology to the Haber-Bosch process, can efficiently synthesize ammonia under ambient conditions and has a reduced carbon footprint. Here we systematically investigate the NRR activity and selectivity of transition metal (TM) single-atom catalyst (SAC) anchored WS 2 monolayers (TM@WS 2 ) by means of first-principles calculations and microkinetic modeling. The construction of the reaction activity trend and the identification of an activity descriptor, namely *N 2 H adsorption energy, facilitate the efficient screening and rational design of SACs with high activity. Manipulating the adsorption strength of the pivotal *N 2 H intermediate is a potential strategy for enhancing NRR activity. Utilizing the limiting potential difference of NRR and the hydrogen evolution reaction (HER) as a selectivity descriptor, we screen three SACs with excellent activity and selectivity toward NRR, i.e. , Re@WS 2 , Os@WS 2 and Ir@WS 2 with favorable limiting potentials of -0.44 V, -0.38 V and -0.69 V. By using the explicit H 9 O 4 + model, the kinetic barriers of the rate-determining steps (0.47 eV-1.15 eV) of the solvated proton transfer on the screened SACs are found to be moderate, indicative of a kinetically feasible process. Microkinetic modeling shows that the turnover frequencies of N 2 reduction to NH 3 on Re@WS 2 , Os@WS 2 and Ir@WS 2 are 1.52 × 10 5 , 8.21 × 10 2 and 4.17 × 10 -4 per s per site at 400 K, achieving fast reaction rates. The coexistence of empty and occupied 5d orbitals of candidate SACs is beneficial for σ donation and π* backdonation, endowing them with extraordinary N 2 adsorption and activation. Moreover, the screened SACs possess good dispersity and thermodynamic stability. Our work provides a promising solution for the efficient screening and rational design of high-performance electrocatalysts toward the NRR.