Formaldehyde is a toxic pollutant that harmful effect on human health. This work focus on the development of a colorimetric sensor to detect formaldehyde solution using composites of polydiacetylene (PDA), thiol-functionalized polydiacetylene (PDA-SH) and zinc oxide (ZnO) nanosheets. The as-prepared PDA/PDA-SH/ZnO vesicles exhibit a blue color, and change to a red color in the presence of formaldehyde solution. With increasing formaldehyde concentrations, the optical absorption spectra show the decreasing of absorption intensity at 640 nm wavelength, and new absorption peak occurred at 540 nm wavelength with increasing its intensity. Additionally, the PDA/PDA-SH/ZnO sensor exhibit high selective to formaldehyde and its color transition is easily observed by the naked-eye. These results indicate a possibility to use PDA/PDA-SH/ZnO sensor for on-site monitoring formaldehyde in an environment.