Kaolinite can take the place of pyrophyllite in the use of pressure transmitting and sealing medium due to its good sealing performance, pressure transmitting property and machining capability. The selected kaolinite sample in this paper is not pure mineral, but it is more suitable for the pressure transmitting and sealing. A new pressure transmitting and sealing medium composed of kaolinite and dolomite is obtained through batching, mixing, shaping, and roasting. With this pressure transmitting and sealing medium, a large number of diamond synthesis tests are conducted. The test results show that the per unit yield of diamond is 19 g, the proportions of crystals whose compression strength and grain size are more than 160 N and 0.30 mm are 32% and 82%, respectively, and the cemented carbide anvil consumption is less than 0.5 g per 1 g of the produced diamond. In addition, the synthesis process observed is stable. Compared with pyrophyllite, the selected kaolinite-dolomite can also achieve considerable effect.