The biggest challenge of exploring the catalytic properties of under-coordinated nanoclusters is the issue of stability.We demonstrate herein that chemical dopants on sulfur-doped graphene (S-G) can be utilized to stabilize ultrafine (sub-2 nm) Au25(PET)18 clusters to enable stable nitrogen reduction reaction (NRR) without significant structural degradation.The Au25@S-G exhibits an ammonia yield rate of 27.5 μgNH3·mgAu-1·h-1 at-0.5 V with faradic efficiency of 2.3％.More importantly,the anchored clusters preserve ～ 80％NRR activity after four days of continuous operation,a significant improvement over the 15％ remaining ammonia production rate for clusters loaded on undoped graphene tested under the same conditions.Isotope labeling experiments confirmed the ammonia was a direct reaction product of N2 feeding gas instead of other chemical contaminations.Ex-situ X-ray photoelectron spectroscopy and X-ray absorption near-edge spectroscopy of post-reaction catalysts reveal that the sulfur dopant plays a critical role in stabilizing the chemical state and coordination environment of Au atoms in clusters.Further ReaxFF molecular dynamics (RMD) simulation confirmed the strong interaction between Au nanoclusters (NCs) and S-G.This substrate-anchoring process could serve as an effective strategy to study ultrafine nanoclusters\' electrocatalytic behavior while minimizing the destruction of the under-coordinated surface motif under harsh electrochemical reaction conditions.