Abstract: The viscosity-temperature characteristics of coal ash have an important influence on the stable slag discharge in the entrained flow gasification process. The viscosity-temperature characteristics of Xiwan coal, Xiaobaodang coal and their blends were studied using a Theta high-temperature rotational viscometer. Combined with the ternary phase diagram analysis of ash components, the coal proportion suitable for entrained flow gasification technology was determined. The results show that the slag of Xiwan coal is crystalline slag, and when the temperature is lower than the critical temperature T_c,the viscosity of the slag increases sharply, causing difficulty in slag discharge. By Xiaobaodang coal is added to Xiwan coal, the acid-aluminum ratio and silica-aluminum ratio of ash gradually decrease, the ash composition changes from yellow feldspar to calcium feldspar, and the ash slag form changes from crystalline slag to glass slag, and the optimal silica-aluminum ratio is 2.35-2.97,the acid-aluminum ratio is 1.05-2.08 and the calcium-iron ratio is 0.70-1.98.With the content of Xiaobaodang coal in the blended coal exceeding 30%,the temperature difference corresponding to the ash slag in the viscosity of 2.5-25.0 Pa·s is higher than 100 ℃,and the slag type change tends to be stable. At the same time, the gasification process requires that the critical temperature of ash slag T_c is not in the temperature range of corresponding temperature when the viscosity of ash is 2.5-25.0 Pa·s in order to ensure the stability of slag discharge, therefore, the ratio of the coal blends of Xiwan coal and Xiaobaodang coal suitable for entrained flow gasification applications is 3∶7 or 7∶3.