Al₂O₃ content will inevitably rise in blast furnace slag as iron ore with high alumina content is continually used, and then metallurgical properties of the blast furnace slag will be certainly impacted. In order to research the effect of MgO content on the desulfurization ability and the desulfurization dynamics of the blast furnace slag with high alumina content, we studied the slag with MgO content of 5%, 9% and 13%, respectively. The results show the MgO content has different effect on the viscosity and the desulfurizaton ability of the slag with high alumina content, and can greatly change the desulfurization rate. The higher the MgO content is, the lower the viscosity and the greater the desulfurization speed is. The desulfuration rate reaches the maximum when the MgO content in the slag is 9%. Both the viscosity and the desulfurization ability are preferable when the MgO content is 9% in the slag, the Al₂O₃ content is 17% and the basicity is equal to 1.1.