Abstract: At present, circulating fluidized bed (CFB) units need to participate in deep peak loading and rapid load changing operation, and the load response rate at the initial load changing stage is mainly determined by the heat storage characteristics of the steam-water side. Therefore, a quantitative calculation method for the heat storage of subcritical CFB units in the steam-water side was proposed. The steam-water process of a 300 MW CFB unit with deep peak regulation was segmented according to the characteristics of the steam fluid. The heat storage coefficients of steam fluid and metal at each section in different load operating conditions (30%-100% load) were calculated respectively by combining with design data and actual operating parameters of the boiler. Considering the influence of steam turbine thermal efficiency, the sustainable time of heat storage and load response characteristics of CFB boiler were analyzed. Results show that the heat storage coefficients of the unit in the steam-water side increase with the decrease of the load, and the change rate is larger under 50% load. Considering the difference of the unit operating stability margin, the sustainable time of heat storage utilization in the steam-water side decreases, and the load response capacity of the unit decreases significantly with the decrease of load.