Coal ash slag viscosity is the most important parameter for the design and the stable running of entrained flow gasifier. Iron serves as the most abundant variable-valence element in coal ash slag, but the effects of iron in coal ash slag on viscosity and slag crystallization under the gasification conditions are not available. In this research, the iron transformation behaviors are discussed, and predictive models for iron transformation based on the ash chemical composition, atmosphere, slag residence time and temperature are established. In the fully liquid slag, Fe2+/Fe3+ increases with SiO2/Al2O3 (mass ratio) and temperature, but it decreases with the increasing iron content, a value (alpha = CaO/(SiO2/Al2O3 ), mass ratio), and oxygen partial pressure. The equilibrium and kinetic model considering CO and H-2 diffusion is built for prediction of Fe2+/Fe3+ in slag at given temperature, oxygen partial pressure, and residence time. The model is applied to predict the Fe2+/Fe3+ of Yanzhou coal and it is found that Fe2+ in Shell slag is higher than that in Texaco slag. In partial crystalline slag, the iron transformation is sensitive to the variations of SiO2/Al2O3 and SiO2/Al2O3 because of the formations of melilite and spinel. The prediction models for melilite and spinel transformation are also built, but the result has not been validated due to lack of in-situ results in the commercial gasifier. The above models offer the guidance for the prediction and adjustment of slag behavior in the gasifier, especially for the iron-rich coal.