Considering the absorption taking place during the back and forth passages inside of the opticl films, and the optical films being "thermally thin" in photoacoustics, a photoacoustic expression for optical thin films is deduced according to the one-dimensional heat-flow piston model. The amplitude of photoacoustic signal is proportional to the absorptivity of the film. Some photoaooustio absorption spectra of films on glass are measured, which is in agreement with the theoretical expressoin. This photoaconstic spectral meiihod shows high sensitivity and precision, particularly Hi table for measuring tlie alosorpiiion spectra of weakly absorbing films.