This study discovers a mechanism called redox-promoted Brønsted acid activation using DFT calculations through mechanistic elucidation of the phenol and cyclohexadiene annulation catalyzed by the AuCl3/AgOTf mixed system. According to this mechanism, triflic acid (HOTf) is likely to be the active catalyst generated in situ as a result of the irreversible reduction of gold(III) to gold(I). The corresponding annulation reaction proceeds through two linked catalytic cycles, the first of which conducts the hydroarylation of diene with phenol and is significantly faster than the second, which produces the observed product via intramolecular cyclization. The [OTf]-counteranion of HOTf is found to play an important role in preventing the polymerization of cyclohexadiene. To confirm that HOTf is the active catalyst in both catalytic cycles of the annulation process, we performed experiments with HOTf as the catalyst and achieved the same product as when AuCl3/AgOTf was used as the catalyst. Additionally, NMR spectroscopy and ESI-MS experiments supported the production of the Au(I) ion and HOTf Brønsted acid.