Iron phosphate is the main material for the synthesis of lithium iron phosphate battery cathode material, which is mainly produced by co-precipitation method of using ferrous sulfate and phosphate salt. The sulfur content in the iron phosphate prepared by the co-precipitation process in the sulfate system is high, which has to be removed by washing with large amount of water of around 60~100 tons per ton of iron phosphate, bringing a huge burden of sulfate-bearing wastewater treatment. To reduce the amount of wastewater from the source, this research proposed an alternative way for desulfurization by high-temperature calcination based on the feature of sulfate decomposition at high temperatures. The thermodynamic feasibility and kinetics of the desulfurization were studied. The results showed that the sulfur in ferric phosphate existed in the form of sulfate, which can be effectively removed by high-temperature calcination. A higher temperature was preferred for a more satisfactory desulfurization efficiency. Kinetic study uncovered that the desulfurization reaction order was 2, and the activation energy was 88.075 kJ/mol, indicating a chemical reaction control mode. The sulfur content in iron phosphate can be reduced to less than 0.01wt% by calcining at 1173 K for 10 min.