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This invention relates to the field of sludge treatment technology, and more particularly to a method for preparing struvite by acid leaching of phosphorus from sludge gasification residue with kaolinite assistance. The method includes the following steps: S1, calcining a mixture of kaolinite and sludge in a certain proportion at 800-1000℃ for a period of time; S2, dissolving the calcined product with an acid solution, separating the solid and liquid, and collecting the phosphorus-containing supernatant; S3, adding a certain amount of magnesium and ammonium sources to the phosphorus-containing supernatant, adjusting the pH to 9.4-10, stirring for a period of time, separating the solid and liquid, washing the filtered solid with deionized water, and drying to obtain struvite crystals. This invention introduces kaolinite before sludge calcination, increasing the phosphorus content in the calcined product, improving the utilization rate of sludge gasification residue, and solidifying heavy metals in the sludge. It avoids a large amount of heavy metals entering the phosphorus-containing supernatant during acid leaching, resulting in a stable and reliable MAP struvite material.