Phase change materials exhibit tunable electrical and optical response, providing rich potential to build active devices with tunable properties. Here, we propose and demonstrate a tunable infrared absorber based on vanadium dioxide (VO2) thin films. Compared with conventional absorbers relying on either nanostructures or Fabry-Perot cavities, our proposed device shows near perfect absorption while having a subwavelength thick absorbing film. Moreover, the absorption intensity can be controlled dynamically around the phase transition temperature of VO2. We model the optical response of the VO2 intermediate states with an effective medium theory to help fitting and understanding the phase change behavior during the phase transition. The calculated electric field distribution as well as the absorption maps are presented to show how the light is absorbed in the thin film platform. The proposed device has the potential for many applications including thin photodetectors, modulators and tunable emitters.