An　adaptive　modulation　and　coding　(AMC)　scheme　based　on　a　Moore　state　machine　is　proposed　for　the　high-speed　railway　communication　systems.　By　setting　a　buffer　zone　for　the　signal-to-noise　ratio　threshold　value,　the　proposed　scheme　is　able　to　reduce　the　frequent　transition　of　the　modulation　and　coding　scheme.　Simulation　results　show　that　the　proposed　AMC　strategy　has　a　more　stable　spectral　efficiency　than　the　traditional　AMC　method　using　a　piecewise　function.