In recent years, ammonia (NH3) decomposition for hydrogen (H2) production has garnered increasing attention in both fundamental research and industrial applications 1,2. Currently, supported Ru-based catalysts exhibit the highest catalytic activity for this reaction. However, even with Ru catalysts, ammonia decomposition still requires relatively high temperatures (400–600°C). Heteroatom doping is a common strategy to enhance the catalytic activity and stability of catalysts. In this work, a systematic density functional theory (DFT) computational study is conducted to investigate the effects of heteroatom doping (N, S, O) on the dehydrogenation steps of ammonia decomposition over Ru single-atom catalysts (Ru-SACs) supported on sumanene. From the energy profiles of the five catalytic reactions, it can be observed that doping with N atoms hardly alters the energy barriers for the dehydrogenation steps in the ammonia decomposition on Ru-SACs supported on sumanene. In contrast, doping with S and O atoms significantly reduces the energy barriers for the first and second dehydrogenation steps in the ammonia decomposition on Ru-SACs supported on sumanene. These barriers even become lower than those on unsupported Ru-SACs. Particularly noteworthy is that doping with O atoms lowers the energy barrier for the first dehydrogenation step of ammonia decomposition from 1.28 eV to 0.53 eV, a reduction of 0.75 eV. All these findings provide deeper insights into ammonia decomposition on supported Ru catalysts, and even on supported transition metal catalysts in general.