A computational study was contributed to explore the origin of stereoselectivity of NHC‐mediated cyclization reaction between benzoic acid and o‐phthalaldehyde for asymmetric construction of phthalidyl ester. The most energetically favorable pathway mainly includes the following steps: (1) nucleophilic attack on carbonyl carbon of o‐phthalaldehyde by catalyst NHC, (2) formation of Breslow intermediate, (3) oxidation by DQ, (4) asymmetric formation of dual C−O bonds, and (5) dissociation of catalyst with the product. The C−O bond formation was testified as the stereoselectivity‐determining step, the R‐configurational pathway is more energetically favorable than the S‐configurational one. The non‐covalent interaction (NCI) and atom‐in‐molecule (AIM) analyses were performed to reveal that the O−H ⋅⋅⋅ O and C−H ⋅⋅⋅ O hydrogen‐bond interactions are the key factors for controlling the stereoselectivity. The detailed mechanism and origin of stereoselectivity give useful insights for understanding organocatalytic reactions for asymmetric construction of C−O bond.