In order to improve the properties of magnesium oxysufate cemented material, the effects of active admixture such as fly ash and slag powder on citric acid modified magnesium sulfate(CMOS) cemented materials and citric acid/sodium silicate modified magnesium sulfate(SCMOS) cemented materials were investigated. The results show that fly ash and slag powder can improve the flexural strength and water resistance of CMOS cemented materials, especially the late flexural strength increases mostly. Adding 10% fly ash or slag powder further improves the mechanical properties and softening coefficient of SCMOS cemented materials and specifically the corresponding 28d immersed softening coefficients increases to be over 085. The micro analyzing results show that the filling effect of fly ash or slag powder makes the hardening structure of CMOS and SCMOS cemented materials more compact; fly ash or slag powder can delay the formation of basic magnesium sulfate phases and decrease the formation of Mg(OH)2 in the hardened structures. It was found that fly ash or slag powder can postpones the hydration process of CMOS and SCMOS cemented material, and reduce its peak of hydration heat.