Abstract: Absorption might be increased due to the impurities introduced by polishing. To differentiate the surface absorptance and bulk absorptance of K9 substrates is of great importance. It is significant for the research of laser resistance, and helpful for the improvement of producing and polishing technique of K9. In this paper, the principle of laser calorimetry was analyzed, and a laser calorimeter according to ISO 11551 was applied to measure K9 substrates. The weak absorptance of K9 substrates with same polishing technique and different thicknesses was characterized. It is found that the absorptance increases nearly linearly with the increasing of thickness. The equation to calculate the surface and bulk absorptances was analyzed, and the surface absorptance in this experiment was 1.2110-5, while the bulk absorptance coefficient was 1.7210-3/cm which was much greater than the former . The conclusion is made that the K9 absorption is mainly caused by the material itself, and improving polishing technique cannot do much to decrease its absorption.