A microreactor system was developed to investigate the cyclization of pseudoionone (PI) catalyzed by sulfuric acid. The conversion of PI and selectivities of α‐ionone and β‐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 β‐ionone and total yield of α‐ionone and β‐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.