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To improve the density of polycrystalline diamond, a study was conducted to investigate the changes in diamond powder under different pressure conditions, including initial loading, cold isostatic pressing, and six-sided die pressing. The study focused on the particle size distribution, powder density, and microstructural rearrangement before and after applying pressure to different diamond powder sizes and ratios. The process involved the initial random arrangement of particles, followed by the filling of fine particles into voids and rearrangement at 220 MPa during cold isostatic pressing. Subsequently, under ultra-high pressure, large particles (G20～30) were crushed and gradually filled the voids. The buffering effect of fine particles resulted in fewer fractures in the dual particle size formula (G2～4 and G20～30) compared to the single particle size formula (G20～30), which facilitated higher stacking density of the diamond powder. These findings provide valuable data support for optimizing the particle size and ratio design of diamond powders for the high-temperature high-pressure (HPHT) synthesis of high-performance polycrystalline diamond composite.