Different synthesis systems of 2,6-Dimethyl-4-(3-nitrophenyl)pyridine-3,5-dicarboxylate 3-(2-methoxy)ethyl 5-(3-phenyl)-2(E) –Acrylic (named as Cilnidipine impurityⅠ) were studied. A one-step aromatization reaction for the preparation of Cilnidipine impurityⅠby using Cilnidipine as the starting material and active MnO2 as the oxidant was reported. The reaction conditions were optimized and the optimal conditions were obtained as follows: m(Cilnidipine) ∶m(MnO2) = 1.00∶1.22, toluene as solvent , reaction temperature at 105 ℃ and reaction time 1 h.. Under the above-mentioned conditions, the product with a yield of 92.0% and a purity of 99.2% (normalization method) was obtained. The structure of product was confirmed by 1HNMR, 13CNMR, HRMS, FTIR. This reaction achieved green, efficient and direct transformation from Cilnidipine to Cilnidipine impurityⅠ.