||Addition of sludge into biomass is an alternative method to capture alkali metals and obtain biomass good combustion conditions. The ash fusion characteristics of sewage sludge (SS), petrochemical sludge (PS), wheat straw (WS), and series of the SS/PS and WS blends in different blending ratios were investigated in this study. The mineral matter transformations during the co-firing of SS/PS with WS were examined. Results indicate that the addition of SS/PS can raise the ash fusion temperatures of WS ash, and the ash fusion temperatures of most SS/PS–WS ashes are higher than those of WS ash. The SS/PS-WS ashes have different mineral matters and microstructures in different blending ratios after the co-firing of SS/PS with WS. It is found that the mineral matters in the SS–WS ash are silicates, silicon dioxide and ferric oxide with high melting points, while the matters in the PS–WS ash are phosphate, manganese aluminate, silicon dioxide, and aluminum nitride with high melting points. The results obtained in this study are not only useful for biomass-fired power stations looking for alternative co-firing fuels but reduce ash deposit formation and agglomeration in biomass-fired boiler.