This study explores how the introduction of high-speed rail (HSR) in China has contributed to a reduction in aviation carbon dioxide (${\mathrm{CO}}_2$) emissions. In line with China's Dual Carbon Policy, the nation has set ambitious goals to achieve carbon peak by 2030 and carbon neutrality by 2060, necessitating comprehensive industrial upgrades across all sectors. Despite the challenges associated with decarbonizing the aviation industry, we conduct empirical analysis to examine the competitive impact of China's domestic high-speed rail network on air transportation and emphasize the environmental benefits of HSR in reducing aviation carbon emissions. Firstly, we construct a panel dataset covering China's domestic air routes and high-speed rail spanning from 2009 to 2019. Our staggered difference-in-differences analysis reveals that the introduction of HSR had a negative impact on China's medium-haul aviation market. Specifically, our analysis estimates that the entry of HSR into the medium-short-haul (600–1000 km) and medium-long-haul (1000–1400 km) air markets resulted in the removal of over 367.8 thousand and 713.3 thousand seats per city pair, respectively. Considering China's power generation mix and carbon emission factors, we calculate that within these two distance ranges, the negative impact of HSR on aviation has resulted in significant environmental benefits. From 2010 to 2019, each city pair with an HSR connection has achieved cumulative savings, averaging 69.57 thousand and 200.69 thousand tons of aviation carbon emissions, respectively. Furthermore, with the optimization of power generation mix and the implementation of dual carbon policy, HSR will continuously contribute to saving 6,436 tons of carbon emissions per city pair by 2035. Prior to the widespread adoption of sustainable aviation fuel, high-speed rail may have played a significant role in reducing aviation carbon emissions. In conclusion, our findings confirm that if China maintains its commitment to decarbonize the power generation industry, HSR holds great potential to further reduce aviation carbon dioxide emissions.