The thermal diffusivity of the materials has been studied by the photoacoustic piezoelectric (PAPE) technique. The thermal diffusivity of yttrium aluminum garnet (YAG) composite material has been studied. The PAPE method based on a simplified thermoelastic theory was proposed. The PAPE theory based on a simplified thermoelastic theory has been analyzed. The experimental system has been set up according to the simplified thermoelastic model. The experimental system was calibrated by investigating the diffusivities of many referential materials. The effective thermal diffusivity of the YAG composite material was investigated by the PAPE technique. The results were shown that the PAPE technique can be used to measure the thermal diffusivity of metal materials accurately.The measurement of YAG’s thermal diffusivity was shown that the PAPE method can effectively measure the thermal diffusivity of new composite materials in the appropriate experimental conditions.