It is thought that the smelting reduction is one of the advanced technology for metallrugical industry. The constitutes and characters change continuously because the charging is loaded periodically. The influence of slag viscosity is important for smelting process. The foundation of viscosity calculating model is propitious to rapidly forcast the slag viscosity. The viscosity calculating model of melting slag is founded according to the ion theory and the characters of iron-bath smelting reduction slag, the calculating program is finished. The result indicates that the calculating value and the measuring value is approaching. This model can be applied to calculate or forecast the iron-bath smelting reduction slag viscosity.