The characterization and comparison of sludge ashes at various ashing temperatures were carried out to study the relationship between the ashing fusibility, physicochemical properties, and thermogravimetric characteristics. The low temperature ashes were black gray and had the characteristic of the coexistence of irregular and rounded particles, which contained a certain proportion of residual unburned combustible material. As the ashing temperature increased, the flake-like crystals transformed into long columns with a compact arrangement. Agglomeration of the ash samples became significant and provided the supporting effect of a skeleton structure. The ash samples at a low ashing temperature had lower ash characteristic temperatures. As the ashing temperature increased, the characteristic temperature increased, which implied that the minerals with high melting points provided the supporting effect of a skeleton structure. The main mineral compositions of the ash samples at the low ashing temperature were quartz, berlinite, magnetite, calcite, calcium pyrophosphate, and muscovite, while at the high ashing temperature, the main minerals were whitlockite, orthoclase, anorthite, anhydrite, and quartz. During the thermogravimetric analysis, the weight loss processes were characterized by a three-stage thermal degradation reaction, as follows: (1) dehydration process and decomposition of light organic volatiles, (2) comprehensive effect from the combustion of unburned combustible residues, the dehydroxylation reaction, and the desulfurization process, and (3) potential anhydrite decomposition. Because phosphorus was chemically combined with calcium preferentially, phosphorus played an important role in the sulfur release, expressed as SO3. When the sewage sludge ashes were rich in phosphorus, anhydrite could be reacted with abundant berlinite, forming whitlockite and aluminum sulfate, and the decomposition of aluminum sulfate resulted in the volatilization of sulfur, expressed as SO3. However, the anhydrite remained constant and the sulfur was difficult to volatilize if there was a lack of phosphorus in the sewage sludge ashes.