Synthetic CVD single crystal diamond is one of the hot spot issues in superhard materials and gemology. Whereas most large single-crystal diamonds synthesized by CVD methods often come with a brown hue. CVD diamonds are often treated by the high temperature and high pressure (HPHT) method to improve the color and transparency. In this paper, based on the previous HPHT experiments, three samples with obvious positive effects are selected, utilizing a series of spectrum tests like UV-Vis absorption spectroscopy, infrared spectroscopy, photoluminescence spectroscopy, three-dimensional fluorescence spectroscopy, laser Raman spectroscopy and X-ray rocking curves to compare. Results indicated that brown and dark brown samples annealed at high temperatures work well. The absorption coefficient of the UV-Vis absorption spectrum of the sample decreases obviously, and transparency has improved significantly. Mid-infrared and near-infrared spectroscopy of CVD diamonds before and after HPHT-processing shows that the absorption peak at 1 332 cm^-1 is related to the N⁺ center, a common feature of CVD brown diamonds. The absorption peak at 3 124 cm^-1 is related to the NVH⁰ defect center, common in CVD diamonds and HPHT treated diamonds. Meanwhile, a set of absorption peaks at 2 700~3 200 cm^-1 was observed, associated with C—H stretching vibration. High temperature has a great influence on the H groups in CVD diamonds. After HPHT treatment at 5~6 GPa 1 500~1 700 ℃, in near-infrared band shows a set of absorption peaks at 4 337, 6 352, 7 354, 7 540, 7 804 and 8 535 cm^-1. Therefore, the appearance of these peaks can indicate that the sample has been treated at a high temperature. This study can provide bases for the identification of CVD diamonds, and HPHT treated CVD diamonds. The results of photoluminescence spectroscopy and three-dimensional fluorescence spectroscopy shows that the concentration of NV^- center decreases while the concentration of SiV^- center increases. Only when the temperature is above 1 500 ℃ at 5~6 GPa, thethree-dimensional fluorescence spectra of the samples were enhanced at λ_ex/λ_em=500 nm/575 nm while the fluorescence peak disappeared at λ_ex/λ_em=490 nm/550 nm. In a sense, the peak changes can indicate that the sample was treated at a high temperature. The phase analysis results show that the Raman shift FWHM at 1 332 cm^-1 and XRD rocking curve FWHM decreases after HPHT-processing, which shows good consistency. The crystallization quality of brown CVD diamonds gets better after HPHT-processing.