We report the thermal failure process of the quantum cascade laser. Firstly, high temperature and strain in the active region are verified by Raman spectra, and the conspicuous catastrophically failed characteristics are observed by scanning electron microscope. Secondly, the defects generate serious structure disorder due to the high temperature of the active region and the resulted strain relaxation. Thirdly, the abundant atomic diffusion in the active region and substrate are observed. The structure disorder and the change of element composition in the active region directly lead to the quantum cascade laser failure. The theoretical analysis fits well with the results of experimental studies.