Nitrogen reduction reactions (NRRs) under mild conditions retain an enormous challenge in effective N-2 activation on metal-based catalysts. Here, by spin-polarized DFT calculations, we systematically investigated the potential of single transition-metal atoms (Sc to Zn, Nb to Cd, W, Pt and Au) supported on the experimentally available WS2 nanosheets with sulfur vacancies as catalysts for N-2 fixation and conversion. The results confirm that the WS2 nanosheet with sulfur vacancies is a good platform for anchoring single transition-metal atoms. Based on proposed several criteria, it is found that the single Ni atoms anchored to sulfur vacancies possess outstanding catalytic activity of NRR. Here, the Ni-WS2 nanosheet acts as likely Lewis-acid system and accepts electrons from the N-2*, which causes the weakening of N-N bond. Reaction energy diagrams of the NRRs show that the whole NRRs on Ni-WS2 surface prefer to occur along the alternating associative pathway, and form the ammonia without the requirement of energy input. Overall, our results reveal that the Ni-WS2 system is a quite promising single-atom catalyst with less energy-consuming and good stability for NRRs. These findings could motivate more experimental and theoretical researches to further explore the potential of single transition-metal atoms anchored to two-dimensional substrate materials for NRRs.