A new approach was proposed to prepare phosphate fertilizers（magnesium ammonium polyphosphate） containing trace elements such as calcium and magnesium through calcination，in response to the problem of difficult utilization of by-product magnesium ammonium phosphate in the production process of lithium iron phosphate（precursor of iron phosphate）.The process and mechanism of preparing magnesium ammonium polyphosphate from magnesium ammonium phosphate were studied under the conditions of calcination temperature of 250~550 ℃ and calcination time of 0.5~3.0 h.The crystal structure，sample quality change during calcination，gas phase products，morphology，specific surface area，porosity，and particle size of the samples were analyzed by thermogravimetric Fourier transform infrared（TG-IR），X-ray diffraction（XRD），scanning electron microscopy（SEM），nitrogen physical adsorption analyzer，and laser particle size analyzer.The diameter distribution was characterized，revealing the mechanism of the reaction process.The experimental results showed that within a certain range，increasing the calcination temperature and prolonging the calcination time could promote the polymerization of phosphate and improve the denitrification rate.Based on the distribution of phosphorus forms and denitrification rate in the product，the optimal calcination temperature was 350 ℃ and the calcination time was 2 h.Under these conditions，the polymerization degree distribution of phosphorus in magnesium ammonium polyphosphate obtained was 1~3，the polymerization rate reached 45.28%，and the denitrification rate reached 79.90%.The obtained magnesium ammonium polyphosphate showed an improvement of over 30% in nutrient indicators such as available calcium，available magnesium，water-soluble phosphorus，and available phosphorus compared to the original magnesium ammonium phosphate.It could be used as a new type of phosphorus fertilizer containing trace elements such as calcium and magnesium.