Objective To solve the capacity decrease problem of laser cutting in surgery, changes in the optical properties of biological tissues during thermal denaturation were studied by using albumen as samples.Methods The integrating-sphere system measured the intensity changes of collimated transmitted light, diffuse transmitted light and reflected light as the samples were heated from 26℃ to 99℃ by a hot air gun at 650nm laser irradiation.The curves of absorption coefficient and scattering coefficient were calculated.Results The temperature’s effect on optical properties of samples was small before thermal denaturation.And during thermal denaturation, transmittance decreased while absorption and scattering coefficients increased.Conclusions Thermal denaturation of tissues at low intensity laser irradiation caused absorbency decreased.Therefore, the laser should be maintained at a high intensity to reach the ablation threshold in surgery.