Abstract: The orthogonal experiment was conducted to investigate the effects of fly ash,cement and mass concentrations on mechanical performances of cemented backfill with fly ash and mullock.By range and variance analysis of the compressive and tensile strength,the influence of the three factors on the compressive and tensile strength of the cemented backfill can be analyzed.The microstructure of cemented backfill with fly ash and mullock was observed by using the SEM.It is concluded that the three factors have significant influence on the compressive strength of cemented backfill at early age,and the dosage of cement is the dominant factor of the long-term compressive strength.At the curing ages of 3,7 and 28 d,the degree of influence on the compressive strength of the cemented backfill is as follows:Slurry mass concentration > fly ash admixture > cement admixture.The only significant factors influencing the tensile strength of the cemented backfill are the cement and fly ash admixture,and the degree of influence on the tensile strength of the filler is:Cement admixture > fly ash admixture > slurry massconcentration.The denseness of the internal microstructure of the cemented backfill increases with the mass of the cement admixture or slurry mass concentration,which in turn shows the increase of compressive and tensile strength of the cemented backfill.