Asymmetric one‐carbon homologation or ring expansion of ketones with formal insertion of carbene intermediate, is a challenging but useful strategy to construct a complex skeleton. Sc(III) and chiral ligands have been employed in this regard. However, due to flexible conformations and a variety of stereo models, the origin of stereochemistry remains ambiguous. Density functional theory (DFT) calculations were carried out to explore the interactions that control the stereoselectivity of a Sc(III)‐catalyzed asymmetric homologation. The trans influence of counterions was found to affect the coordination mode of ketone to Sc(III), and consequently affect the stereoselectivity. DFT study of Sc(III)‐catalyzed asymmetric homologation of ketones with diazo compounds revealed a new coordination mode of substrates which was rationalized by the trans influence. The origin of stereo‐selectivity was explored based on the coordination mode. A promising new strategy to adjust stereo‐selectivity by trans influence was proposed.