Single-atom catalysts (SACs) with active sites of transition metal-nitrogen (TMN_x) centers are efficient for the electrochemical nitrogen reduction reaction (NRR), a sustainable strategy of ammonia synthesis, but the loading of active sites is relatively low due to the clustering of transition metal heteroatoms. Herein, 2D transition metal-porphyrin (TM-PP) sheets possessing high-loading intrinsic TMN₄ centers were investigated to catalyze NRR using density functional theory (DFT). The results suggest that 5d TM-PP sheets are more efficient than 3d and 4d TM-PP sheets owing to their better capability of electron transfer, which is related to the screening effect induced by inner-shell electrons. Among them, as the most promising one, W-PP sheet possesses high mass loading of 38.33 wt% and low overpotential of 0.15 V, beneficial for catalyzing NRR with good selectivity.