In order to improve the operation flexibility of coal-fired power units, a thermal system of coal-fired power system coupled with supercritical carbon dioxide (S-CO₂) energy storage cycle is proposed, and the effect of operation parameters on the system irreversible loss is investigated by thermodynamic exergy analysis. The results show that, the energy storage efficiency of the system can reach 56.14%, and the S-CO₂ flow rate and the S-CO₂ compressor/turbine pressure ratio have a greater influence on the system exergy efficiency. With the increase of S-CO₂ flow rate from 50 kg/s to 70 kg/s, the exergy efficiency of the system increases from 44.0% to 61.0%. With the increase of compressor/turbine pressure ratio from 3.0 to 6.0, the exergy efficiency of the system increases from 27.5% to 52.5%. The method proposed provides a theoretical reference for improving the coal-fired power unit operation flexibility, and provides ideas for large-scale grid connection of renewable energy.