In this paper, by choosing catalyst of FeNiMnCo alloy, boron-doped diamond single crystals are synthesized under 5.1–5.6 GPa and 1230–1600℃; the temperature field is studied by finite element method (FEM). First, the P-T phase diagram for diamond single crystal growth, in the synthesis system of FeNiMnCo-C-B, is obtained, and the lowest synthesis conditions of 5.1 GPa and 1230℃ is found in the studies. By simulation with FEM, it is found that the content of boron element should be less and less in the growth of diamond single crystal in the {111} sector, and the reason is that the growth speed is reduced in the sectors. By growing diamond crystals with {111} faces, it is also found that the content of boron element in {111} secondary sector is greater than that in {111} primary sector, which is duo to the rapid growth of {111} secondary sector. Compared with the synthesis of diamond single crystal by film growth method, the diamond crystals thus obtained has no pits, the doping content of boron can be greater, and the diamond can be synthesized by temperature gradient method.