The invention relates to the technical field of building materials, and discloses a method for preparing a high-performance phosphogypsum-based self-leveling mortar. The method comprises the followingsteps: S1, obtaining raw materials comprising 40-60 parts of phosphorus building gypsum, 15-22 parts of cement, 10-24 parts of fly ash, 3-10 parts of stone powder, 1.5-4 parts of a redispersible latex powder, 0.05-0.15 part of cellulose ether, 0.02-0.06 part of a nano-carbon fiber, 0.3-0.8 part of a water retaining agent, 0.1-1.3 parts of a water reducer, 0.1-0.4 part of an anti-settling agent, 0.01-0.08 part of a defoaming agent and 30-42 parts of fine sand aggregates; and S2, mixing the materials obtained in step S1, stirring the obtained mixture until uniformity, and adding water accounting for 13-15% of the total weight of the mixture and performing stirring until uniformity when the mortar is used. The reinforcement of a phosphogypsum-based self-leveling material and the high fracture toughness of the carbon fiber effectively reduce and inhibit the expansion of matrix cracks, and greatly improve the interfacial pore structure of the self-leveling material. The method of the invention improves the bonding degree of the interface of the phosphogypsum matrix, increases the toughness of the phosphogypsum matrix and increases the overall strength.