A simple trimethylsilyl trifluoromethanesulfonate (TMSOTf)-promoted intermolecular cascade reaction of aromatic diazo ketones with olefins has been developed. This method directly gave 3-phenylethylideneoxindoles from 3-diazooxindoles and styrenes with exclusive regioselectivity, chemoselectivity, and E-stereoselectivity. The key to the success of the reaction and higher yields is the elegant use of TMSOTf, which gradually released the active triflic acid promoter in situ. The reaction tolerates a wide substrate scope of 3-diazooxindoles and styrenes with electron-donating and electron-withdrawing groups and works well on the gram scale.