In the process of leaching magnesium oxide by water containing CO2, the obtained concentration is normally only 5 g/L to 12 g/L. To obtain concentrated magnesic leaching liquor from boron mud, pressurized dissolution processes were investigated under various CO2 pressures from 1 MPa to 5 MPa, as well as ratios of solid to liquid, stirring speeds and leaching temperatures, which significantly influenced the leaching of magnesium oxide. And the concentration of MgO in the solution was achieved as high as 26.63 g/L with MgO leaching efficiency of 38.80% under the condition of CO2 pressure 5 MPa, solid/liquid ratio 0.18, stirring speed 500 rpm, leaching temperature 50 degrees C, and leaching time 1.5 h, meanwhile, the impurity concentrations of CaO, B, Fe, Pd and Si were 0.16 g/L, 0.11 g/L, 0.23 g/L, 0.01 g/L and 0.06 g/L, respectively. The magnesium oxide leaching method by pressurized carbonated water is applicable for treating with other materials rich in magnesium hydrate or magnesium oxide, such as calcined magnesite, dolomite and boron mud by CO2-soda process.