Abstract: To obtain the influence of feed water flow rate, coal supply flow rate and other parameters on the load cycling capacity of the subcritical unit, a dynamic thermal system model was established and verified by taking a 300 MW unit as an example. Under 75% load working conditions, the change trends of drum pressure and temperature, main steam pressure and temperature, and unit power generation in the thermal system were studied after the feed water flow rate was reduced by 2.5%-10.0% and the coal supply flow rate was increased by 2.5%-10.0%, and the temporal and spatial distribution characteristics of heat storage in the thermal system were obtained. Results show that when the feed water flow rate decreases or the coal supply flow rate increases, the drum water level drops, the steam production of the evaporation system increases rapidly, and the output power of the unit increases. In the subcritical unit, the heat storage of the drum boiler accounts for more than 90% of the entire thermal system. Taking the lowest drum water level of 100 mm as the safe operation boundary, the total heat storage of the thermal system decreases by a maximum of 9.2 GJ when the feed water flow rate decreases, and decreases by a maximum of 9.4 GJ when the coal supply flow rate increases. During the evolution of the heat storage of each part, the heat storage in the drum has the largest change amplitude, and the heat storage change accounts for about one-third of the total heat storage, which has the potential to increase the load cycling rate by 1.85%/min.