Ultrafine catalysts of CuB and Me-CuB were prepared by chemical reduction, examined for hydrogenolysis, and co-used with potassium methoxide for both carbonylation and one-step synthesis of methanol from CO/H-2 at low temperature. Doping with Th and Cr additives enhanced the stability and dispersion of copper in the resulting catalysts, Th-CuB and Cr-CuB, and promoted the hydrogenolysis of methyl formate. The Th-CuB and Cr-CuB catalysts, co-used with potassium methoxide, prevented the deactivation of potassium methoxide in carbonylation and in one-step synthesis of methanol. A higher temperature favored the hydrogenolysis of methyl formate, but limited the formation of methyl formate by carbonylation. A higher P-CO facilitated the formation of methyl formate, but inhibited the hydrogenolysis of methyl formate. A trade-off between temperature and P-CO must be made in the one-step synthesis of methanol, including carbonylation and subsequent hydrogenolysis. The optimal temperature was around 423 K. A higher PCO was employed to overcome the limitation of methyl formate formation. An accompanying rise in P-H2 (H-2/CO greater than or equal to 2) was required to weaken the inhibitory effect of CO on the Cu-based catalysts during the subsequent hydrogenolysis of methyl formate.