Abstract It is difficult to utilize the large amount of gasification slag produced in the coal chemical industry, which is a major problem in solid waste resource utilization. The residual carbon from coal gasification fine slag (RC) was actived by KOH wet method. The activation temperature (110℃, 130℃, 150℃, 170℃) was tuned to regulate the pore structure of the RC and explore the effects on the electrochemical performance. When the active temperature was 150℃, the electrochemical performance of the as-actived RC-150 exhibited the best among the actived materials. The specific capacitance of RC-150 reached 109.0 F/g at a current density of 0.5 A/g. The energy density of the symmetric supercapacitor using RC-150 as active materials reached 3.78 Wh/Kg at a current density of 2 A/g. After 10 000 cycles, the capacitance was able to maintain 103.81% of the initial value, showing better cycle stability. The results could be attributed to the improvement of pore structure, the removal of oxide impurities and the modulation of graphitization degree during the activation process.