Abstract: The effects of three supplementary precursors,namely slag（GGBFS）,fly ash（FA）and silica fume（SF）,on the properties of alkali-activated steel slag were studied. The mechanical properties of the samples were analyzed by the compressive strength,flexural strength and flexural to compressive strength ratio of the paste and mortar. The microstructure of the samples was characterized by thermogravimetry（TG）and scanning electron microscopy（SEM）. The results show that the mechanical properties of alkali-activated steel slag increase with the increase of slag content. The optimal content of fly ash is 20% and the optimal mixture of silica fume is 5%（mortar）or 10%（paste）. The brittleness law obtained from the flexural to compressive strength ratio is as followed:granulated blast-furnace slag＞silica fume＞fly ash＞pure steel slag. The quantity of hydration product C—S—H determines the mechanical properties and microstructure. Its internal micro-cracks caused by drying shrinkage can impair its long-term flexural strength. The mechanical properties and brittleness should be considered in the mix design of alkali-activated steel slag cementitious materials. The powder modification experiments of different auxiliary precursors on alkali-activated steel slag provide a reference for improving the comprehensive utilization efficiency of steel slag.