Abstract: The absorptance of the optical coating was quantitatively measured by the lock-in thermography method. The periodically modulated laser intensity was absorbed by the coatings to be tested, and a thermal wave was formed on the coatings surface. The thermal distribution signal recorded by the infrared camera was processed by the lock-in method to obtain the thermal image with improved SNR. The standard absorption samples were used to calibrate the system to obtain the quantitative relations between the photothermal signal amplitude and the samples absorptance, then the samples to be tested were measured under the same experimental conditions, and the absolute absorptance could be obtained by the direct calculation of the photothermal signal. The experimental study is carried out at the wavelength of 1 060 nm, the absorptance of the Nb₂O₅ coating with different thickness was measured, and the measured absorptance can reach to 80 ppm.