Abstract: Effects of atmosphere, mixing ratio and residue carbon on ash fusion characteristics of biomass and bituminous coal were investigated. The pine sawdust ash was mixed with Wuhai bituminous coal ash at different mass ratios. The ash fusion temperatures (AFTS) of the mixed ash in different atmospheres were determined by ash-melting point measuring device. X-ray diffraction ray was used to investigate the reason for AFTs variation from the perspective of minerals evolution. The result shows that AFTs of mixed ash in weak reducing atmosphere is lower than that in oxidizing one due to formation of fayalite and hercynite. In addition, the difference in AFTs under reducing and oxidizing atmosphere is related to the content of Fe in the mixed ash. The increment of pine sawdust ash favors generation of gehlenite, akermanite and leucite ect., which can be classified as low temperature eutectics and help to reduce the AFTs of mixed ash. Moreover, the AFTs of the mixed ash increase initially and decrease afterwards and then increase again with the increment of residue carbon content in coal ash, due to formation of Fe-C eutectic (Fe_xC_y), local reducing atmosphere and skeleton action of residue carbon.