In this paper, gem diamond synthesis is systematically studied using the multiseed method in China-type cubic high-pressure apparatus. High-quality Ib diamond crystals are synthesized in a growth cell with 3-5 diamond seeds, by adjusting the growth cell assembly and investigating the pressure and temperature regions of diamond synthesis. Because of several diamond seeds embedded in a growth cell, the synthesized diamond crystals possess the same morphology and quality. At the same time, the whole growth rate increases apparently. Using the multiseed method of diamond synthesis the growth cell volume can be effectively utilized, the efficiency of diamond synthesis can be enhanced, and the problem of low utilization rate is solved. Meanwhile, those also provide an effective support for the gem diamond industrialization synthesis.