Abstract: As the requirements for conversion rate and production efficiency in coal conversion industry further improving,the thermal conversion process of coal tends to be performed in converter with high temperature and high pressure. In the high-temperature and highpressure combustion furnaces and gasifiers with slag tapping,the minerals in coal are completely melted into slag and then discharged. For an entrained-flow gasifier with slag tapping and a water-cooled wall,the melting temperature of coal ash is required to be lower than the operating temperature,and the viscosity of the molten slag ranges from 2.5 to 25.0 Pa·s. Moreover,the fluctuation of viscosity with temperature should be small in the operating temperature range. The melting temperature and the viscosity-temperature characteristics of coal ash during gasification are the key factors affecting the flowing of slag. The development of traditional testing methods for ash melting temperature was reviewed and analyzed in this paper. The principle of standard methods in various countries is to determine the melting temperature by the deformation of the compacted sample during the heating process. However,the melting temperature alone cannot provide enough information to achieve fine control of modern large-scale gasification processes,and the full-stage testing of the coal ash melting process helps to guide the actual production more accurately. A few qualitative and quantitative studies of the melting process by researchers in various countries show that the deformation temperature in the melting temperature is not the real temperature at which the coal ash starts to melt. In view of the deposition and sintering problems of coal ash,the start temperature of shrinking and the initial formation temperature of the liquid phase that predicted by thermodynamic calculation helps to predict the temperature at which coal ash may deposit or sinter more accurately. At present,the test of viscosity-temperature characteristics still relies on the high-temperature rotational viscosimeter,which is time-consuming and complex. Therefore,a simpler and time-saving method is used to quickly screen the viscosity-temperature characteristics of suitable feedstocks. In addition to the experimental methods,the application of simulation calculation methods is more and more widely used in the study of coal ash fluidity. Through thermodynamic calculations and molecular simulation methods,it is possible to obtain mineral composition and microstructure changes of the melt that are difficult to measure in experiments. The non-equilibrium method in molecular dynamics can simulate the shear thinning process of complex fluids more accurately,thereby the obtaining viscosity calculation results are closer to the experimental values. Not only does the use of non-equilibrium methods improve the accuracy of the calculation results,but also increases the complexity of the calculation and the time required for the calculation. At present,there are not many choices of calculation models for the coal ash system. Therefore,the complexity of the system and the accuracy of the calculation results should be considered comprehensively when using the simulation calculation methods. With the further study of the melting process and the general application of simulation calculation methods,the mechanism of the change for macroscopic property presented in experiment will be easier to be clarified through the microstructure changes,which in turn will help optimize existing simulation calculation methods and parameters.