In this paper, Surface Acoustic Wave (SAW) delay line (DL) based humidity sensors are optimized. These sensors comprise two ultrasonic interdigital transducers (IDT) for generating and detecting SAW. The SAWs propagate along a piezoelectric substrate covered with a polyvinyl alcohol (PVA) layer that is sensitive to humidity. PVA layers are conventionally used in capacitive, resistive or acoustic resonance humidity sensors because of their sensitivity to humidity and low fabrication cost. The PVA layers are generally prepared with 10 wt% and their sensitivity range is from 10% to 90% relative humidity (RH) at room temperature. However, some applications require sensors capable of measuring RH greater than 90%. Therefore, in this study, several PVA layers with different mass percentages were considered to optimize and increase the sensitivity range of these ultrasonic sensors. It is shown that a 5 wt% PVA layer has a sensitivity range from 10% to 98% RH with good repeatability, as well as hysteresis of less than 0.5%. Finally, the mechanisms underlying the response of SAW humidity sensors are also discussed and analyzed.