Gasified fine ash (GFA) exhibits poor reactivity due to its low carbon content and high ash content, which restricts its potential for combustion utilization. To enhance the reactivity of GFA, we employed a circulating fluidized bed (CFB) as a thermal modification unit (TMU) to thermally modify GFA from different entrained-flow gasifiers. We also experimentally investigated the impact of equivalence ratio, temperature, and feedstock moisture content on the thermal modification of GFA. After thermal modification of GFA, the pore structure of modified fine ash (MFA) was significantly improved, the proportion of active sites in residual carbon increased, and the proportion of graphitized structures decreased. An appropriate increment in the equivalence ratio increased the number of carbon active sites in MFA. A good thermal modification effect was found on the GFA produced by different types of gasification furnaces. The temperature rise promoted gasification reaction in the TMU and restricted carbon conversion, thus decreasing the consumption of combustible residual carbon. An increase of water content in the GFA improved the pore structure, but excessive water caused adverse effects. Display omitted •Increased combustion performance of GFA is achievable through thermal modification.•Studied impact of ER, T, and M modification conditions on combustion performance.•Mechanism of thermal modification to enhance reactivity of GFA has been elucidated.