The thermal diffusivity rates of 8 different thickness SrTiO3 films are measured based on the back excitation photoacoustic technology,the variety of photoacoustic signal amplitude,phase and the thermal diffusion rate with the film thickness is studied. It is found that the photoacoustic signal amplitude and phase gradually decrease with the increase of film thickness,while the thermal diffusion rate of the thin film increases first, then decreases, and in a certain thickness,the thermal diffusion rate reaches its maximum.On the condition of the same thickness, the research on the relativity between the photoacoustic signal amplitude, phase,film thermal diffusivity rate and the modulation frequency shows that the photoacoustic signal amplitude and phase non linearly decrease with the increase of frequency,but the thermal diffusivity rates of the thin film are stable in different modulation frequency.