Tapered optical fiber-based interferometric sensors have potential applications in some fields for the measurements of solution concentration, refractive index, biomolecules, etc. By coating specific functional film onto the surface of a tapered optical fiber, it is possible to further extend the detection range and enhance the measurement sensitivity. In this paper, a tapered optical fiber sensor based on Mach-Zehnder modal interference is studied. An atactic polystyrene thin film was used as a functional layer to coat onto the surface of the tapered region. Detection of aqueous glucose of different concentrations was carried out using the tapered fiber sensor with or without the coating. The experimental results show that the measurement sensitivity can be significantly improved by coating the atactic polystyrene thin film onto the tapered fiber with suitable waist parameter.