A colorimetric sulfur dioxide (SO2) gas sensor based on a core-shell composite was developed. The composite was fabricated with a silicon dioxide core and a mesoporous MCM-41 shell ([email protected]), and further loaded with a mixture of zinc chloride (ZnCl2), sodium nitroprusside (SNP) and hexamine as an SO2 indicator. The sensing properties of [email protected] toward SO2 were measured in solid powder, discs and a gas detection tube (GDT), respectively. Each of these sensing configurations showed a distinct color change from pale yellow to red, which indicates good potential for naked-eye detection of SO2. The limit of detection (LOD) is 2 ppm for [email protected] discs, which indicates high sensitivity to SO2. The performance of GDT suggested a linear relationship between the SO2 concentration and the response length of the red portions in a range of 100-1000 ppm. This work shows promising potential of [email protected] as an easy, effective and rapid response sensing material for the in situ detection of SO2.