Area of optical glass damaged by laser is only several hundred microns.Its transmittance detection is difficult with common method.A kind of new test and algorithm is reported in this paper.A metallographic microscope and an optical fiber spectrometer are mixed into a test system with lenses.The system is useful to quantitatively analyze the damaged micro-area and morphology.At the same time,the transmittance spectra of the micro-area can be tested before and after damage.These spectra are calculated to gain the transmittance.At last,the damage threshold is obtained by spectrum transmittance.