Owing to the need for a hydrostatic high-pressure cubic large cavity (hexahedral top) press used in high-pressure research and production of superhard material, two kinds of pyrophyllite powder compacts (A and B) from pyrophyllite mine in South Africa are prepared, and compared with the domestic yellow pyrophyllite powder compacts (Mentougou, Beijing) produced by the same process, to establish experimental methods and physical criteria for evaluating the pressure transmission and sealing performance of pyrophyllite. During the experiment, standard pressure materials such as Bi, Tl, and Ba are used to in-situcalibrate the pressure at the central positions and sealing edges of the pyrophyllite pressure chambers from the three aforementioned compacts under normal pressure conditions. Additionally, the silver melting point method is employed to obtain the corresponding relationship between chamber pressure at high temperature and system loading when using these three types of pyrophyllite as load-transmitting sealing materials. The results show that under the same hydraulic pressure loading, the difference in pressure at the central position between South African pyrophyllite B powder blocks and domestically produced pyrophyllite powder blocks does not exceed 0.1 GPa. Furthermore, in pressurization process and depressurization processe, the differences in pressure between the central position and the sealing edge of the pyrophyllite blocks are notably similar. Compared with South African pyrophyllite A powder blocks, pyrophyllite B powder blocks exhibit a closer resemblance to domestically produced pyrophyllite powder blocks in terms of high-temperature load transmission and sealing performance. Pyrophyllite B powder blocks from South Africa have the potential to serve as a substitute for domestically produced pyrophyllite without changing the existing superhard material synthesis process, making them promising candidates for use as load-transmitting media and sealing materials. These research findings hold significant academic importance in the realms of high-pressure research and superhard material production. They provide valuable insights into the selection of suitable transmission and sealing materials and the optimization of high-pressure experimental conditions. Additionally, this study presents robust method and criteria for experimental procedures and performance assessment.