The work aims to investigate the failure of the diamond infrared windows at high temperatures due to graphitization. The 1500~1800 ℃ thermal shock test was carried out to CVD free-standing diamond films by plasma arc generator. After double-sides polishing, optical microscopy, X-ray diffraction (XRD), Raman spectroscopy and Fourier transform infrared spec-troscopy (FTIR) were used to characterize the diamond films treated by thermal shock. X-ray photoelectron spectroscopy (XPS) was applied to analyze the relationship between the evolution of the carbon bonds and the shock temperatures. After thermal shock, the graphite of the diamond film mainly existed in the boundaries and the graphitization process accelerated as the treating temperature increased and then resulted in the loss of IR transmittance. General two-peak fitting and improved three-peak fitting methods were compared in the deconvolution of C1s core level. By introducing the AC component, the problem that ΔEB (the difference between the sp3 and sp2 peak positions) was not fixed has been solved. The appearance of the AC component, which mainly existed in the diamond film reconstitution surfaces and the grain boundaries where the atomic arrangement was disorder, was caused by the fact that the carbon atoms did not form a perfect equivalent sp3 electron orbital hybridization. The rationality of the three-peak fitting was demonstrated by comparing the relationship between the percentage of AC and sp3 component. According to the evolution of bonding characteristics and IR absorption, the estimated activation energies of diamond graphitization were consistent, which were 227 kJ/mol and 250 kJ/mol, respectively. Graphitization along grain boundaries is the main cause for the loss of IR transmittance of CVD diamond films after thermal shock. The relationship between shock temperature and IR transmittance as well as sp2 carbon content is established.