With the development of the laser towards high-power and high energy, laser-induced damage threshold of optics becomes one of the important parameters to evaluate the laser damage resistance of optics. Therefore, accurately measuring of the laser-induced damage threshold optics become the focal point studied. And the key to accurately measuring of the laser-induced damage threshold is whether the laser-induced damage can be accurately identified when it occurs. In order to solve low accuracy, long testing time, narrow scope of applications and complex operation of the common damage identification methods, a new testing method to diagnose the laser-induced damage of optics, called plasma diagnosis, is proposed in this paper. Based on this new method, the testing platform was set up, and the spectrum obtained by fiber spectrometer was analyzed under laser radiation by different laser energies. Take whether the spectral lines of the feature element contained in the measured optics occur as standard. The laser-induced damage threshold of K9 glass has been tested, and the test result was compared to that measured by the plasma flash method and the microscope method. The results show that, the plasma diagnosis method proposed in this paper has high-accurate judgment, high-testing speed, simple testing equipment, and easy to realization, which can greatly improve the testing efficiency of the laser-induced damage threshold of optics.