Recently, a variety of carbon materials can be turned into pure polycrystalline diamond directly without any additives under extreme high pressures and high temperatures (pressure above 13 GPa and temperature above 2000 ℃). Polycrystalline diamond shows a broad application prospect because of its superior performance. However, it is difficult to realize the industrialization of pure polycrystalline diamond on current high pressure equipment due to the high synthetic conditions. The focus of our work is that the synthesis of pure polycrystalline diamond can be realized in the same synthesis range of single diamond produced from the solvent metal (pressure below 6 GPa and temperature below 1500 ℃). The carbon materials can precipitate from the solution in a form of diamond, and fill into the gaps between the diamond particles. According to some domestic scholars' researches on polycrystalline diamond, the solvent method can reduce the high temperature and high pressure conditions on which carbon may transform into diamond directly, and precipitate from the solution in the form of diamond into the gaps between diamond particles. Through a deep study of the approach, the low addition content, even pure polycrystalline diamond without gaps can be prepared. In this paper we have prepared pure polycrystalline diamonds under relatively lower conditions (the pressure being below 6 GPa and the temperature below 1500 ℃) by the method that the metal solution layer infiltrates into the gaps between the pure diamond particles and then the diamond particles will grow up. We also carry out a research on the mechanism of carbon transformation in the preparation of polycrystalline diamond. Compared with the traditional method of powder mixing technology, the melt infiltration and growth method is more advantageous to prepare high abrasive resistance and high density pure polycrystalline diamond.In order to prepare pure flawless polycrystalline diamonds without additives by China-type large volume cubic high-pressure apparatus (CHPA) (SPD-61200), we study thoroughly on the melt infiltration and growth method under high pressures; and this provides a theoretical guidance for pure polycrystalline diamond synthesis. In this paper, polycrystalline diamond is prepared by melt infiltration and growth method at pressures below 6 GPa and temperatures below 1500 ℃. Mechanism research of carbon transformation is made under high pressure and high temperature (HPHT). Through the analyses of optical microscope, X-ray diffraction, and field emission scanning electron microscope measurements, graphitization occurs on the surface of diamond in the procedure of metal solution infiltrating, and then the generated graphite quickly change into diamond-like carbon under HPHT. Meanwhile, the morphology of diamond particles changes distinctly in the syntheses process. From the analysis of experimental phenomena, carbon may undergo three transformations in the preparation: 1) graphite is generated due to the graphitization on the surface of diamond particles, which is caused by the metal solution infiltrating; 2) the generated graphite quickly fills into the gap with the form of diamond-like carbon during the sintering stage; 3) the diamond-like carbon is dissolved in a metal solution, and then precipitates between particles in the form of diamond. The mechanism research on carbon source transformation plays an important guiding role in the industrialization of no-additive, no-gap pure polycrystalline diamond preparation.