Fine-tuning of the coordination environment of single-atom catalysts (SACs) is effective to optimize their catalytic performances, yet it remains challenging due to the vulnerability of SACs. Herein, we report a new approach to engineering the coordination environment of M-N-C (M = Fe, Co, and Ni) SACs by using glutamic acid as the N/C source and pyrolysis atmosphere as a regulator. Compared with that in N₂, NH₃ was able to promote the doping of N at T < 700 ℃ yet etch the N-species at higher temperatures, by which the M-N coordination number (CN) and the electronic structure were delicately tuned. It was found that the electron density of Ni single atoms increased with the decrease of Ni-N CN. As a consequence, the capability of Ni-N-C to dissociate H₂ was greatly enhanced and a higher catalytic activity in chemoselective hydrogenation of functionalized nitroarenes was achieved. Moreover, this modulation method could be applied to other transition metals including Fe and Co. In particular, the as-synthesized Co-N-C SAC afforded a turnover frequency of 152.3 h⁻¹ with 99% selectivity to 3-vinylaniline in the hydrogenation of 3-nitrostyrene, which was the highest ever reported thus far and was at least one order of magnitude more active than state-of-the-art noble-metal-free M-N-C catalysts, demonstrating the great potential of engineering the coordination environment of SACs.