Kiln phosphoric acid (KPA) technology could produce P2O5 with high purity and has been applied in thermal phosphoric acid industry;however the formation of fouling in the high-temperature rotary kiln restricts the sta-ble and long-term operation. In this paper, the reaction of phosphate ores with gaseous P2O5 was investigated in a high-temperature reactor, and the CaO–SiO2–P2O5 tary phase diagram was analyzed to understand the foul-ing formation mechanism. The results showed that the low-melting-point products, such as Ca(PO3)2 and Ca2P2O7, are responsible for the fouling in the KPA process. In addition, a small amount of impurities, e.g., alumi-num and iron, could facilitate the generation of the low-melting-point products and cause serious fouling. Based on the high-temperature SiO2–P2O5 and CaO–SiO2–P2O5 phase diagram analysis, the control of Si/Ca molar ratio (e.g., Si/Ca=2.0) was found to avoid fouling formation in the kiln. These results could provide the operation parameters of reaction temperature and feeds composition to suppress the fouling in the kiln reactor for the phosphoric acid production in industry.