Abstract: The process of causticizing -precipitating scheelite by calcium hydroxide from sodium tungstate solution is the technical way of the realization of recycling NaOH reagent for tungsten ore decomposition ,and wolframite and scheelite and wolframite mixed ore green smelting. This paper studies equilibrium constant and standard free energy change of causticizing -precipitating scheelite by calcium hydroxide from sodium tungstate solution reaction at different temperatures, and analyzes the effect of the activity coefficient of sodium hydroxide and sodium tungstate on the reaction equilibrium. The research shows that the effect of temperature and sodium tungstate solution concentration on precipitation rate of scheelite is very significant. When the calcium hydroxide dosage is 1.4 times of the theoretical amount, temperature 100℃, tungsten concentration of sodium tungstate solution is 105 g/L, the holding time is 2 h, the stirring speed is 350 r/min, the precipitation rate of scheelite can reach above 96%. Comparing with traditional process of calcium chloride used as a precipitating agent, calcium hydroxide precipitating scheelite requires larger times of the theoretical amount, longer reaction time and faster stirring speed .