Abstract: In the process of building the new type power system, coal-fired power plants need to undertake the task of peak load regulation, and continuous low-load operation has gradually become the norm. The dynamic characteristics of supercritical boilers under full load conditions have been extensively studied, but little research has been done on the dynamic characteristics under low load conditions and the position of the phase transition point of the working fluid. In order to study the dynamic characteristics of the once-through boiler during deep peaking, a dynamic simulation model of supercritical once-through boiler was established in Dymola platform, and the variations of steam temperature, main steam flow rate and the position of the phase change point of the evaporation system were investigated under 50% THA load after the boiler's fuel quantity, feed water flow rate, feed water temperature and total air flow rate were perturbed respectively. The results show that when the fuel quantity, feedwater flow rate, feedwater temperature and total air flow are all step increase 5%, the changes in the main steam temperature are 46.89, −39.03, 19.74 and −6.29 ℃, respectively, and the response time is 2 210, 1 520, 2 180 and 710 s. Maintaining the water-to-coal ratio is still the primary task for ensuring the low load steam temperature stability, and the change of feedwater temperature is not conducive to the stability of steam temperature. The height of the evaporation section gradually increases after the increase of feedwater flow rate and total air flow, the evaporation section gradually shortens after the increase of fuel flow rate, and the evaporation section first lengthens and then shortens after the increase of feedwater temperature. Among them, the effect of fuel flow on the height of evaporation section is the most significant. Under low load conditions, the intermediate point temperature and steam flow of the once-through boiler fluctuations are more intense, which is not conducive to the safe and stable operation of the boiler, and has higher requirements for automatic control.