SO2 sensor is highly demanded in the application fields such as environmental protection and food manufactory. Herein, an integrated microsensor is developed for SO2 gas detection. For the fabrication of microsensor, ZnO nanosheets sensing material is firstly loaded onto the sensing area of the microsensor by using inkjet printing technology. Then, Al2O3 loaded with Ru nanoparticles (Ru/Al2O3) as catalyst are locally deposited onto the surface of ZnO nanosheets layer. Gas sensing performance measurements indicate that the fabricated microsensor exhibits a selective response to SO2, and a good linear relationship in the range of 5 to 115 ppm SO2 gases. Besides, this integrated microsensor has short response and recovery time. On-line mass spectrometry (on-line MS) experiment further reveals the formation of sulfur monoxide (SOradical dot) radical as an intermediate product for SO2 sensing. During the sensing process, Ru/Al2O3 as catalyst layer brings SO2 molecules to be broken down into easily detectable species (i.e. SOradical dot), and then ZnO nanosheets with abundant gas transport channels capture the produced SO generating output signals. Therefore, this kind of sensing material configuration exploits the advantage of each element, and makes it possible for trace and selective detection of SO2 gas.