The Inhibition of impurities of As, P, Si and Sn in the caustic decomposition of tungsten concentrates was studied. It indicated that in the presence of scheelite (CaWO₄) and Ca(OH)₂, during caustic decomposition of sdheelite with NaOH, impurities, such as As, P, Si, will be more completely inhibited in the cake in the forms of NaCaAsO₄, Ca₃(AsO₄)₂, Ca₃(PO₄)₂，Ca₅(PO4)₃(OH)，CaSO₃ and CaSnO₃. It has been proved in laboratory experiment and industrial practice that the content of As and Si in the Na₂WO₄ solution from caustic decomposition of scheelite and wolframite mixed concentrates is only 20%〜30% of that from caus-tic decomposition of wolframite concentrates with the same contents of WO₃ and impurities.