To interpret sum frequency generation (SFG) vibrational spectra collected from adsorbed protein molecules using structures of the interfacial proteins, the thin film model has been evaluated both theoretically and experimentally. The calculations indicate that nonlocal contributions to SFG spectra of interfacial proteins usually can be ignored. The experimental results indicate that for some polarization combinations, such as ssp and sps, local field variations across the adsorbed protein layer will not substantially affect SFG signals. Therefore, SFG spectra collected from adsorbed protein molecules are generally dominated by the electricdipole contribution. From such spectra, quantitative structural or conformation information about interfacial proteins can be deduced using the thin film model. As an example, we analyzed the orientation distribution of the methyl groups of BSA molecules at different interfaces.