Excessive Al₂O₃ will worsen the performance of blast furnace slag (especially fluidity) and bring a series of adverse effects on normal smelting of blast furnace. In order to study the influence of Al₂O₃ on the structure and properties of molten blast furnace slag, molecular dynamics method is used to systematically study the microstructure and viscosity changes of molten blast furnace slag with different Al₂O₃ mass fractions, and the microstructure informations such as radial distribution function (RDF), coordination number (CN), bond length, bond angle distribution function and mean square displacement (MSD) are analyzed based on BMH potential function. The results show that Si and Al in the melt mainly exist in the form of [SiO₄]^4- and [AlO₄]^5- tetrahedrons. With the increase of Al mass fraction, the coordination number values of Si-O and Al-O gradually approach 4, which indicates that the structures of [SiO₄]^4- and [AlO₄]^5- tetrahedrons become more and more stable. The bond lengths of Si-O, Al-O, Ti-O, Mg-O and Ca-O in the melt are 1.59, 1.77, 2.35, 2.29 and 2.31 A, respectively, and the bond lengths of each bond have no obvious change with the increase of Al₂O₃ mass fraction. The bond angles of O-Si-O, O-Al-O, Si-O-Si and Si-O-Al fluctuate around 109°, 104°, 145°, 135°, respectively, and the peak height and peak width of the O-Si-O bond angle distribution curve remain the most stable, which once again indicates that the [SiO₄]^4- tetrahedral structure in the blast furnace slag melt is basically not affected by Al mass fraction, and it is the most stable skeleton structure in the blast furnace slag network structure. The mass fraction of bridging oxygen Si-O-Si decreases, the mass fraction of Al-O-Si and Al-O-Al increases, and the mass fraction of non-bridging oxygen Si-O-Ca, Al-O-Ca, Si-O-Ti, Si-O-Mg, Al-O-Ti and Al-O-Mg decreases. The self-diffusion coefficient and viscosity of each atom in the melt with different Al₂O₃ mass fraction are calculated by using the relation function between mean square displacement (MSD), Einstein equation and viscosity. The results show that with the increase of Al₂O₃ mass fraction, the self-diffusion coefficient of each atom gradually decreases, and the viscosity gradually increases. When Al₂O₃ mass fraction exceeds 16%, the viscosity increases slowly.