The compressive strength at early and late age is relatively low for low-alkalinity cement prepared by ordinary Portland cement, silica fume and fly ash for the solidification of radioactive incineration ash. In this study, polyaluminum sulfate was used as additive to stimulate the reactivity of pozzolanic materials in the low-alkalinity cement. The effect and mechanism of polyaluminum sulfate on the early age hydration of low-alkalinity cement were characterized by evaluating the compressive strength, porosity, pH value, phase assemblage and microstructure of low-alkalinity cement after addition of polyaluminum sulfate. The results indicate that the incorporation of polyaluminum sulfate significantly improves the early age compressive strength of the low-alkalinity cement. The exothermic peak of early age hydration is advanced and the pozzolanic reaction is promoted. The amount of hydration products increases, and the microstructure is densified. The early age alkalinity effectively reduces, which is beneficial to inhibit the corrosion reaction of metal aluminum in the incineration ash waste form.