In order to investigate coloration genesis and color change mechanism of HPHT synthetic diamond in a dynamic process, the authors conducted a HPHT experiment on yellow diamonds synthesized under the condition of high temperature and high pressure. IR spectra, low temperature PL spectra and DiamondView^TM images tested before and after the HPHT treatment show that, due to the limitations of the synthesis method, HPHT synthetic diamond has various kinds of defects, such as stress defect, vacancy and impurity elements like N and Ni. These defects greatly affect the quality and color of HPHT synthetic diamonds. The color centers of the tested samples before and after HPHT experiment are (N-V)^-, (N-V), H3, C center, (Ni-V)^-, Ni-N complexes and Ni^-. The experimental physical conditions (7 GPa, 1 480℃) we adopted contribute to accumulation of nitrogen in diamond crystal. And this accumulation reaction leads to the transformation of diamond type from Ⅰb (with a bit A aggregation)to Ⅰb+ⅠaA mixed type and is also helpful to the change of synthetic diamond's color.