Abstract: For theoretical issues of powellite soda selective leaching, equilibrium diagrams of Ca- W-Mo-CH₂O vsystem were drawn on the basis of thermodynamic data at 25 ℃. Total concentration of ions （W, Mo）-pH relationship diagrams were depicted. The effects of system pH value and total carbon-containing ions concen -tration on mixed system selective leaching were analyzed. Along with the concentration of CO₃^2- increasing, the total （W, Mo）concentration in the solution was added. The saturation concentration of Mo is about 1.85 times that of W. Under a certain total carbon concentration, CaMoO₄ is more likely to decompose than CaWO₄. In addition, the high pH value should be maintained to avoid an unexpected abundant of HCO₃^- instead of CO₃^2- mainly existing under low pH value, leading to the decrease of W and Mo leaching rate. By controlling the total carbon concentration and pH value of the solution, Mo can be obtained by the effects of W and Mo selective leaching.