•A novel humidometer depended on unsymmetrical U-shaped twisted MFC is reported.•The special MFC sensing structure are formed by two microfiber tapers.•A layer of PVA material with tens of nanometers is covered on the coupling region by the "first in the middle and then on both sides" coating method.•The unsymmetrical U-shaped MFC sensing structure possesses a large detection range (18%-95%RH) at the laboratory temperature. Owing to the integration of microfiber couplers (MFCs), polyvinyl alcohol overlay (PVA) material and U-shaped structure, a novel humidometer depended on unsymmetrical U-shaped twisted MFC is reported in this work. The special MFC sensing structure is formed by two microfiber tapers, which attach together under the action of van der Waals force and electrostatic attractive force, therefore it is convenient to adjust the coupling length according to the design requirements. In addition, a layer of PVA material with tens of nanometers is covered on the coupling region by the coating method "first in the middle and then on both sides". Finally, the U-shaped MFC is fabricated. This sensing structure possesses a large detection range (18%–95%RH) at the laboratory temperature. The demonstrated sensing structure with larger effective coupling length shows that the humidity sensitivity is 100.2 pm/%RH with a very good linearity (R2 = 0.9961). The RH sensitivity is enhanced by ˜2 times compared with that of the sensor with shorter effective coupling length. This sensor also shows other advantages, including high reversibility, repeatability and low temperature cross-sensitivity.