In this paper, high-quality colorless diamonds were successfully synthesized by temperature gradient method (TGM) in China-type cubic anvil high-pressure apparatus (CHPA) at 6.0 GPa and 1340 degrees C. High-purity graphite was used as carbon source and the Fe-Ni alloys with different Fe contents were used as the solvent-catalyst for diamond synthesis. The effects of Fe content (high-Fe-type and low-Fe-type) on boron concentration were specially investigated. All the colorless diamonds exhibit the typical boron-related absorption peaks of 2802 cm(-1) in infrared (IR) absorption spectra, but the boron concentrations are different. Compared with the low-Fe-type alloy solvent, it is more difficult for boron atoms to incorporate into diamond lattice in high-Fe-type alloy solvent system. This work provides a new approach to control the boron concentration in colorless diamonds and synthetic high-purity diamond crystal.