Abstract: In view of the features of fly ash as abundant resources，low costs and improving the construction of mortar，the influence of fly ash on strength and liquidity of slag cemented materials is studied through experiments. The results show that the slag cemented backfilling materials with the lime and desulfurization gypsum as the activator，the compressive strength of backfilling body gradually declined with the adding amount of fly ash increasing. 1% of fly ash is added each time，the intensity decreased by 1.82% at 7 d and by 1.61% at 28 d on average. Meanwhile，the yield stress and viscosity of the backfilling is getting lower，which indicates that fly ash can largely improve the rheology of backfilling materials，and make high concentration of mortars conveying by gravity flow. Comprehensively considering the strength and the rheology，the optimal dosage of the fly ash ranges from 10% to 20%. 

Key words: Fly ash, Cemented materials, Filling