The modulation frequency affects the thermal diffusion depth in sample during the coatings absorption measurement. The absorption value and thermal defects in various depth of the sample can be measured using proper modulation frequency. Theoretical analysis of the impact of modulation frequency on thermal diffusion depth at certain thickness of the coatings is presented. The results show that the thermal diffusion depth is shorter while the thermal conductivity is smaller and the coating is thicker,meanwhile the influence of the substrate is weaker. When the thickness of the coatings does not change,the thermal diffusion depth is shorter while the modulation frequency is higher,and the photothermal signals of different substrates tend to be same.