The humidity detection performance of a Love wave sensor coated with polymer film was reported. A polyvinyl alcohol(PVA) layer was spincoated on an ST90°-Xquartz substrate as the waveguide layer and absorbing layer. When the relative humidity in the closed test container was increased from 9% to 88%, the working frequency and insertion loss of the device were measured. Experiments showed that both frequency shifts and insertion loss increments were caused by the mass load of absorbed water vapor. The variations followed the III adsorption isotherm.