Recently, single-atom catalysts (SACs) are receiving significant attention in electrocatalysis fields due to their excellent specific activities and extremely high atomic utilization ratio. Effective loading of metal atoms and high stability of SACs increase the number of exposed active sites, thus significantly improving their catalytic efficiency. Herein, we proposed a series (29 in total) of two-dimensional (2D) conjugated structures of TM 2B 3N 3S 6 (TM means those 3d to 5d transition metals) and studied the performance as single-atom catalysts for nitrogen reduction reaction (NRR) using density functional theory (DFT). Results show that TM 2B 3N 3S 6 (TM = Mo, Ti and W) monolayers have superior performance for ammonia synthesis with low limiting potentials of −0.38, −0.53 and −0.68 V, respectively. Among them, the Mo 2B 3N 3S 6 monolayer shows the best catalytic performance of NRR. Meanwhile, the π conjugated B 3N 3S 6 rings undergo coordinated electron transfer with the d orbitals of TM to exhibit good chargeability, and these TM 2B 3N 3S 6 monolayers activate isolated N 2 according to the “acceptance–donation” mechanism. We have also verified the good stability (i.e., E f < 0, and U diss > 0) and high selectivity (U d = –0.03, 0.01 and 0.10 V, respectively) of the above four types of monolayers for NRR over hydrogen evolution reaction (HER). The NRR activities have been clarified by multiple-level descriptors (ΔG *N2H, ICOHP, and Ɛ d) in the terms of basic characteristics, electronic property, and energy. Moreover, the aqueous solution can promote the NRR process, leading to the reduction of ΔG PDS from 0.38 eV to 0.27 eV for the Mo 2B 3N 3S 6 monolayer. However, the TM 2B 3N 3S 6 (TM = Mo, Ti and W) also showed excellent stability in aqueous phase. This study proves that the π-d conjugated monolayers of TM 2B 3N 3S 6 (TM = Mo, Ti and W) as electrocatalysts show great potentials for the nitrogen reduction.