Abstract: Four coal samples including Shenmu coal from Shaanxi, Nalingou coal from Inner Mongolia, Yangchangwan coal from Ningxia and Nantun coal from Shandong were prepared for the research on the rheological properties of coal ash at high temperatures. The high temperature rotational viscometer was used to study the effect of shear rate on the shear stress and viscosity of coal ash. The results indicate that the coal ash slag falls into a Newtonian fluid when the temperature is above the liquidus temperature, while it gradually turns into non-Newtonian fluid when the temperature is below the liquidus temperature. Shenmu and Yangchangwan coal ash owns the plastic and shear-thicking characteristics, while Nalingou and Nantun coal ash has the pseudoplastic and shear-thinning characteristics. Meanwhile, the crystals are formed with the decrease of temperature, which makes the yield stress increase and the flowability of coal ash slag decrease.