Abstract: To solve the problem of mechanism of potassium preparation with K-feldspar and phosphogysum, with K-feldspar, calcium sulfate and calcium oxide as raw materials, the thermal response process of system was analyzed under the different molar ratios (n (K-feldspar) ∶ n (CaSO₄·2H₂O) ∶ n (CaO) = 1∶ 1∶ (2 ~ 16) ) . The experiments show that the most suitable proportion of material is 1∶ 1∶ 10, under the batching system, there are three kinds of calcium silicate in the calcined products, including CS, C₂S and C₃S, and their composition and proportion are associated with the reaction temperature of system. Besides, there is only one form (C₂AS) of sialic acid calcium salt, C₃A is nonexistent. These are inconsistent with the reported. There are two different reactions to displace generated K₂O, when the temperature is below 1 100 ℃, the replacement reaction occurs between KAl Si₃O₈ and CaO, and when the temperature is higher than 1 100 ℃, CaO would react with KAl Si₃O₈ and KAl Si₂O₆ respectively. Furthermore, when the temperature is below 1 200 ℃, K₂O is not combined with sulfate radical to sulfate, but escaping as gas. When the temperature is higher than 1 200 ℃, soluble potassium exists as K₂S₂O₈, and K₂SO₄ is unfound. TG-DSC experimental results show that theinitial displacement reaction temperature of the system is about 1 000 ℃, and the reaction goes into overdrive when the temperature is higher than 1 100 ℃. The results are consistent with the XRD analysis of the product which calcined at different temperatures. The cause of system weight loss at high temperature is that K₂SO₄ is converted into K₂S₂O₈ by releasing gaseous K₂O.