The complex chemical composition of coal slag separated out different types and shapes of crystals during the cooling process. The morphology of the crystal in the molten slag influenced the viscosity of the coal slag. The aim of this paper was to obtain the viscosity model of crystalline slag using a new calculation method. The model of the suspension viscosity obtained by the previous study was used to predict the viscosity of molten slag. The correction factor (beta) was introduced in the model of the suspension viscosity. The solid phase volume fraction (phi) in the molten slag was calculated by FactSage. The liquid phase viscosity (eta(0)) was fitted by the viscosities of molten slag in high temperature section. Ten kinds of crystalline slags were applied in this study to prove the accuracy of the modified viscosity model. The viscosity of molten slag predicted by the model (CSM) agreed with experiment data.