Abstract: In order to realize large-scale resource utilization of municipal solid waste （MSW） incineration fly ash in the field of building materials, the effect of the addition ratio of fly ash, water-solid ratio and addition of admixtures on the performance of fly ash-cement expansion cementitious system was investigated, and the hydration products were analyzed microscopically. The results indicated that with the increasing content of MSW incineration fly ash, the dispersion degree of fly ash-cement slurry decreased, the setting time was prolonged, and the compressive strength decreased continuously. The expansion rate increased first and then decreased and reached maximum when the fly ash content was 60%. As the water-solid ratio of slurry increased, the dispersion degree, the setting time and the expansion rate of fly ash-cement slurry all increased, a more reasonable water-solid ratio between 0.37 and 0.40 should be considered. Al and aluminium nitride （AlN） in fly ash reacted with water to generate gas, leading to expansion of the cementitious material, NaOH increased the alkalinity of the solution, promoting the generation of more bubbles, and improved expansion of cementitious materials. CaCl₂ promoted the hydration reaction of the fly ash-cement cementitious system, leading to the formation of "cloud" shaped C—S（A）—H gel, the gel micropores were filled with clusters of hydrated calcium aluminate chloride crystals, making the structure of the solidified body more compact, the compressive strength of the solidified body was improved, and the leaching content of heavy metals and dioxine concentration decreased.