The change of oxygen was studied for the TZM molybdenum alloys with addition of active metal element and carbon after vacuum sintering. The mechanism of deoxidation of molybdenum alloys was analyzed through the thermodynamic calculation. The results show that in the sintering process of powder metallurgical TZM, there are two kinds of deoxidation mechanisms. One is that metal oxide compound in TZM react with the carbon to produce CO and metal carbide; the other is that the disproportionting reaction of MoO2 occurs at high temperature to produce metal Mo and MoO3 gas and the gas is drawn out by the vacuum system. Improving the vacuum degree of the furnace and reducing the partial pressure of product gas can decrease reaction temperature and is good for deoxidation.