Abstract: This paper studies the Mo-H₂O system, W-H₂O system balanced reaction and ε_T-pH calculation formula based on the main electrochemical reaction and the thermo-dynamic data of the reaction component in Mo(W)-H₂O system.The Mo-H₂O system and W-H₂O system potential -pH diagram are mapped out through the thermodynamic calculation at room temperature(298 K).In the graphics, the thermo-dynamic stable region of various substances forms of tungsten and molybdenum under different pH conditions and the possibility of chemical reaction are indicated.The electrochemical properties difference of MoO₄^2-and WO₄^2- are pointed out.The result shows that the potential value of W-H₂O system is significantly lower than the value of Mo-H₂O system.The phenomenon of the potential value differences can be used to select the appropriate reducing agent for selective reduction.The separation of tungsten and molybdenum can be efficiently practiced.