Optical glass is an important optical component in high power laser system,and it is very important to measure its damage threshold.The laser damage test experiments were carried out with the same batch of K9 glasses.The laser damage thresholds were tested by plasma flash,image analysis and spectral analysis.Experimental results show that single testing method can not accurately determine the damage threshold,but The combination of image analysis and spectral analysis can be more reliable to get damage threshold.The measured damage threshold of K9 in 1064 μm wavelength is 22.47 J/cm2,which verifies the reliability of the combined testing methods.