The photoacoustic (PA) angular spectrum method was developed on the basis of attenuated total reflection (ATR) and PA spectroscopies. This method was used to obtain the resonance absorptioin spectrum of Langmuir-Blodgett (LB) films on silicon substrates. The rigorous electromagnetic theory in a layered medium is also presented to explain the experimental results and to characterize the films. By fitting theoretical results to the experimental spectrum, the refractive indices and the thickness of the films can be evaluated. Moreover, as the intensity of the laser beam increased, additional peaks were observed in the angular PA spectrum, predicted to be related to the optical non-linearity of LB films. Compared with conventional ATR spectroscopy, PA ATR spectroscopy has the advantages of a simpler optical system and higher sensitivity, as well as a wider application range.