Abstract: To fully exploit the intrinsic energy storage potential of subcritical units in supporting rapid load variation performance, a dynamic simulation model of unit thermal system and an energy and exergy storage calculation model were developed focusing on a 330 MW subcritical unit. Furthermore, a rapid load variation scheme for the feedwater flow throttling was proposed, and five different system configuration adjustment schemes for utilizing internal energy and exergy storage of unit were analyzed. The support capacity of each scheme for improving the load variation rate of the unit was quantified. Results show that with decreasing load, the energy and exergy storage of thermal system decrease by 19.6% and 23.1%, respectively. The energy and exergy storage of working fluid in the evaporation system account for the largest proportion of the boiler system, representing 51.3% and 52.0% respectively. Feedwater throttling most markedly improves the flexibility of unit, and the supportable rapid load variation duration and maximum power increment are 114.7% and 168.2% higher, respectively, than those achieved with high pressure control valve regulation, and furthermore it maximizes the utilization of the internal energy and exergy storage of unit after feedwater flow throttling.