In the present paper, by the temperature gradient method, the gem-diamond single crystals with B2O3-added in the synthetic system of the FeNiMnCo-C are synthesized under 5.3-5.7 GPa and 1200-1600℃. The P-T phase diagram of diamond single crystal growing in the synthesis system of the FeNiMnCo-C-B2O3, is obtained. By B2O3-added in synthesis system, the V-shape section for the diamond growth, which is the region between the solvent/carbon eutectic melting line and diamond/graphite equilibrium line under pressure and temperature, is moved upwards. We find that the minimum pressure of diamond growing increases from 5.3 GPa to 5.4 GPa and the synthesis range of the low temperature hexahedron diamond growth becomes wider, which can be due to the chemical energy increase of the carbon depositing in the diamond surface by the additive of the B2O3. The synthetic diamond single crystal exhibits a perfect hexahedral shape or cubo-octahedral shape or octahedral shape. In the system of the FeNiMnCo-C-B2O3, we think that the catalyst activity decreases with the generation of CO2, so high-quality diamond single crystal can hardly be synthesized when the content of the B2O3 is more than 3 wt‰ and synthesis time is more than 20 h, However, when the content of the B2O3 is no more than 1 wt‰, the rate of finished products of the low temperature hexahedron diamond will increase significantly. Because the amount of the B2O3 additive is so small, in the syntheses of B2O3-added diamond single crystals, the black areas which appear when B element enters into diamond crystal lattice are not observed. The growth rate of diamond single crystal will be reduced obviously by B2O3-added in synthetic system. Under our system synthesis, when the growth time is 10 h, the growth rate of the diamond will reduce 0.22 mg/h by 2 wt‰ B2O3 added in synthetic system. When the growth time extends to 20 h, the growth rate increases to 0.47 mg/h. Moreover, with the extension of growth time, the catalyst activity decreases continuously with the product of the CO2 increasing in the reaction chamber, so the effect of additive on the growth of diamond strengthens gradually. The results of the scanning electron microscope images indicate that the surface defects of the diamond crystal increases by the addition of B2O3.