Abstract: The temperature fields in the sample and adjacent media are obtained by two mathematical transforms, the analytical solution of temperature field in gas media is given when thermal diffusivity is assumed to be exponentially changed slowly. The phase expression of photoacoustic signal is obtained by using microphone detection theory. When two surface diffusivities are known, the distribution of thermal diffusivity can be determined by relation of photoacoustic signal phase and frequency. It's feature are experiment simple and calculation small. The relevant theoretical derivation and simulation are given in this paper.