The conventional alkaline solution activator has high corrosion and viscosity, which limits the large-scale application of alkali-activated technology in fly ash disposal. It is very important to select appropriate solid activator for the preparation of cementitious materials. One-part alkali-activated slag-fly ash composite cementitious material was prepared in this paper. The effects of different solid activators on the mechanical properties and hydration products of cementitious material were discussed in order to find a more effective fly ash treatment solution. The results show that sodium silicate pentahydrate can improve the early strength of cementitious material, and the additional silicon source promotes the formation of silicon-oxygen tetrahedron, which is conducive to the formation of network hydrated calcium aluminosilicate gel. Calcium oxide has a significant effect on the growth of later strength. The maximum compressive strength of cementitious material at 28 d is 28.90 MPa. The higher alkalinity can make the silicon-aluminum phase continue to dissolve and produce more effective polymerization. The heavy metal leaching concentrations in the cementitious material activated by sodium sulfate, sodium silicate pentahydrate and calcium oxide all meet the standard allowable values. The dense gel structure is conducive to the fixation of heavy metals, reducing cracks and pores, and further reducing the leaching amount of heavy metals.