After studying various material behavior in the diamond synthesis processes and by measuring the factors in the HP/HT apparatus, we summarized the characteristics of diamond growth as follows: diamond grows on the graphite (G)-metal catalyst (Me) material interface. Since the resistance R(G)R(Me), temperature T(G)T(Me) and the heat exchange with outside, et al., the pressure and temperature gradients thus created in the cell will become the driving force of diamond growth. Too large or too small a driving force will be harmful to diamond growth. On the surface of each diamond crystal grain, there is a thin metallic layer of thickness about 15~20 m. This metallic thin film can both act as the carbon carrier as well as the catalyst. The growth of a coarse grain of high-strength diamond requires a stable synthesis system. The paper analyzed the property of such a system and proposed a way to establish the system.