A stabilization treatment was developed for heavy metals in fly ash from municipal solid waste incineration using the heavy metal chelator diisopropyl dithiophosphate potassium (DDP). The mechanism and effect of the DDP chelator treatment on heavy metals in the fly ash was also studied, along with the form transformation rules of the heavy metals after DDP chelator treatment. The results show that 1% DDP achieves a stabilization rate of over 95% for Pb, Zn, and Cd. The effect of DDP was better than that of inorganic stabilizers such as sodium sulphide and lime. The heavy metal concentrations in the leachate after the treatment were lower than those required by the Pollution Control Standards for Hazardous Waste Landfill (GB18598-2001). At pH 1-13, the heavy metal concentrations in the fly ash leachate were far lower than those using the inorganic stabilizers sodium sulphide and lime. DDP retains its stabilizing effect under a broader pH range. After stabilization treatment, the heavy metals in the exchangeable fraction and those bound to carbonates were mainly transformed into those bound to organic matter. This process decreases the unstable content and reduces the risk of secondary pollution of the stabilized products in the environment.