Acoustic resonant spectroscopy technique for measuring the acoustic impedance, ultrasonic velocity and density of micron-scale polymer films is developed. The method observes the acoustic resonance between water, film and the tungsten plate at the frequency range of 20-70MHz, where the interface between the examined film and the plate is vacuum-sealed, and it enables us to characterize a low-density polyethylene film with low acoustic impedance of about 1.9MNm^<-3>s and a poly (vinyl chloride) film as thin as about 8μm.