Abstract: The landfill disposal of municipal solid waste incineration (MSWI) fly ash requires a large amount of cement or chelating agents. In order to reduce the dosage of cement or chelating agents and alleviate the pressure of fly ash storage capacity, sulfuric acid modified kaolin samples were prepared. The ability of sulfuric acid modified kaolin and synergistic chelating agents (SDD) to stabilize Pb in fly ash was studied, and compared with that of cement and SDD. The chemical forms of Pb, functional group characteristics, phase composition, and microstructure changes in modified kaolin stabilized fly ash samples were explored. The results showed that the acidity and specific surface area of kaolin were increased when modified by sulfuric acid, and the modified kaolin had a good long-term stabilization effect on Pb in fly ash. When the addition of modified kaolin was 13.04% in fly ash, it could achieve the same stabilization effect as cement or SDD, and the addition of modified kaolin was 24.46% lower than the cement amount under the same condition. In addition, modified kaolin had good compatibility with SDD, and when they worked together to stabilize fly ash, the amount of SDD added alone could be reduced. During long-term storage (150 days), the chemical forms of Pb in modified kaolin stabilized fly ash samples were predominantly residual. As the storage time increased, the modified kaolin underwent a hydration reaction with fly ash, and the hydration product Ca—Al(Si)—H whisker increased, which enhanced the consolidation of Pb and greatly increased the stabilization of Pb.