A microreactor system was developed to investigate the cyclization of pseudoionone (PI) catalyzed by sulfuric acid. The conversion of PI and selectivities of alpha-ionone and beta-ionone were explored under various conditions. The effects of residence time, molar ratio of acid/PI (M-ratio), reaction temperature, and sulfuric acid concentration on the reaction were evaluated. Sulfuric acid concentration was found to play a vital role in the reaction system, which could greatly influence the conversion and selectivities. A high yield of beta-ionone and total yield of alpha-ionone and beta-ionone were achieved under optimized conditions. The formation of by-products was determined in detail and the side reactions could be restrained by optimizing the reaction parameters.