The Raman spectrum test was performed on the high temperature and high pressure (HTHP) single crystal diamond as the candidates of the substrates in the CVD(Chemical Vapour Deposition, CVD) epitaxial growth, and the crystalline quality of the substrates was judged by the full width at half maximum (FWHM) of the peak. X-ray in-situ measurement study of the substrate lattice changes during the heating (room temperature to 1 000 ℃) and cooling (1 000 ℃ to room temperature) processes. Experiments show that the lattice constant of the substrate changes with temperature, and the stress caused by the lattice change at 1 000 ℃ is close to the GPa level. The lattice constant is larger in the cooling process than in the heating process, and the same phenomenon is found in the calculation result of the linear expansion coefficient. According to the FT-IR test results of the tested sample, it is inferred that the reason for the variant above-mentioned lattice change is the difference in nitrogen content in the sample. The higher the nitrogen content, the larger the FWHM of the Raman spectrum, and the lattice constant of substrate more obvious changes with temperature, the larger the linear expansion coefficient.