Existing studies have not yet discussed the synergetic decarbonisation effects among supply chain firms driven by artificial intelligence (AI) applied in leading focal firms, nor have they explored the corresponding mechanisms in depth. Focusing on the supply chain, this paper constructs a dataset with "focal firm - associated firm - year" observations, to examine the external spillover effects of AI on supply chain decarbonisation. Our findings reveal that AI employed by focal enterprises significantly contribute to curbing carbon emissions among associated enterprises within the supply chain, primarily through green technological innovation (GTI). The role of substantive GTI is predominant, whereas symbolic GTI appears to have an insignificant impact. Meanwhile, from the perspective of synergy, the vertical interaction effect of the supply chain and the horizontal peer effect of industry can strengthen the supply chain decarbonisation effect of AI technology. This paper provides a new synergetic insight for fully releasing the decarbonisation potential of AI based on supply chain interaction, providing empirical evidence and policy references to promote industrial green inclusive development and realize the global zero-carbon goal.