Abstract: The substrate effects on the measurement of weak absorption of optical thin films at low frequency are analysed in theory. The absorptions of different thin films, including single-layer HfO2 and ZnO thin films with different thickness, on two substrates (BK7 and silica), as well as absorptions of blank substrates have been measured by surface thermal lensing technique. As a calibrated sample, the absorptions of singal-layer Ti3O5 thin films were measured by Lambda—900 spectrometer. The experimental results show that the substrate of standard film is the same as the substrate of sample in calibration for low measurement is necessary. After considering the influence of film thickness，the correction would be possible.