In combination with Huimin high-phosphorus iron smelted by smelting reduction and HIsmelt ironmaking technologywhich have the advantages of high efficient dephosphorizationCaO-SiO2-Al2O3-MgO-FeO-P2O5 slags with basicity R (mass ratio of CaO to SiO2)ranging from 0.8 to 1.4Al2O3 ranging from 6.4% to 15.4%P2O5 ranging from 0 to 3% and the content of MgO and FeO fixed as 4% and 6% respectivelyare studied.The slags are synthesized by reagent grade.The mineral composition and microstructure of the slags are studied with scanning electron microscope and the effects of changes in the composition of smelting reduction slags on its viscosity are studied by RTW-10 melt comprehensive physical properties measuring apparatus.Results show that the mineral structure of slags are mainly composed of melilite(gehlenite and akermanite)which is characterized by square and thick skeleton form.The viscosity decreases with the increasing of the slag basicity at fixed P2O5 or Al2O3 mass fractionincreases with the increasing of Al2O3 mass fraction at fixed slag basicity or P2O5 mass fractionand increases with the increasing of P2O5 mass fraction at fixed slag basicity or Al2O3 mass fraction.