Abstract: Circulating fluidized bed combustion technology is a clean coal combustion technology that has witnessed rapid development in recent years. During rapid load changes in circulating fluidized bed units, substantial bed material remains within the furnace, containing unburned residual carbon and unreacted calcium-based desulfurizers. These substances not only affect the boiler’s load change rate but also influence pollutant emissions. Therefore, how to effectively utilize the active reserves within the boiler plays a crucial role in improving the boiler’s heat release rate and enhancing its load response speed. A 350 MW supercritical CFB boiler was selected as the research subject. Numerical simulations were conducted on the periodic fire suppression and startup process of the CFB boiler. Dynamic models for the active residual carbon inventory and active limestone inventory were established to analyze the gas-solid flow and active inventory during the periodic fire-down and restart process. The results indicate that after the secondary air fan and induced draft fan are shut down, the boiler enters a sealed state. In this state, the falling-back speed of the material in the furnace accelerates. The material in the return leg returns to the furnace under the influence of gravity. The average particle volume fraction in the dense phase zone reaches 0.45, while the particle volume fraction in the return chamber increases to 0.52 following material deposition. During the periodic fire suppression and startup process, the active residual carbon inventory under the stable operating condition stage 1 is 6353.11 kg, and the oxygen volume fraction is 5.35%. After starting the secondary air fan and coal feeder, the oxygen volume fraction drops significantly, while the active residual carbon inventory increases to 2868.76 kg. Regarding the periodic fire suppression and startup process, the desulfurization efficiency under the stable operating condition is 59.58%, and the active limestone inventory is 2702.29 kg. After starting the secondary air fan and coal feeder, the active limestone inventory rises to 1482.93 kg, and the desulfurization efficiency reaches 50.35%.