A hybrid optical microcavity from rolled-up polymer/oxide/polymer nanomembranes presents its excellent capability for environmental relative humidity detection. When exposed to a moist surrounding, poly(acrylic acid)/poly(ethylenimine) polymers swell greatly due to the absorption of water molecules, which thus leads to an increased wall thickness of the tubular optical microcavity and therefore presents a profound wavelength redshift of its whispering-gallery mode resonance. These experiments fit well with the calculation based on the Mie-scattering theory. Theoretical calculation also demonstrates that the thin walls of our tubular microcavities contribute to a high detection sensitivity compared to other microcavities. Our work could lead to new designs and applications of optical microcavities.