As one of the key process of flexible power generation, long-term stable low load operation technology plays an important role in the continuous expansion of renewable energy grid connection. It is very common to adopt the operation mode of uneven coal feeding in order to reduce pollutant emissions or adjust bed temperature in the process of load regulation of circulating fluidized bed (CFB) boiler, and this mode put forward higher requirements for the design and operation of CFB boilers. At the same time, the flue gas temperature at the outlet of CFB boiler is low at low load. How to ensure that the steam parameters reach the rated value is one of the urgent problems to be solved to realize the flexible operation of CFB boiler. The parameters of two 350 MW supercritical CFB boilers under coal cut-off and low load operation conditions were deeply calculated and analyzed, and the methods to stabilize the reheat steam temperature at low load were studied. The results show that when the boiler operates under coal cut-off conditions with partial coal feeding ports interrupted, the bed temperature, furnace outlet flue gas temperature, furnace outlet oxygen content and high-temperature reheated steam temperature are basically positively correlated with the coal feeding rate at each coal feeding port. However, the coal feeding deviation and bed temperature deviation have little effect on the outlet steam temperature deviation of water-wall because of the positive flow response characteristics of the water-wall of supercritical CFB boilers. Based on the above research, a boiler transformation and operation optimization technical scheme for stabilizing the outlet steam temperature of high-temperature reheater in supercritical CFB boiler under long-term peak shaving and low load operation was proposed, which could ensure the basic stability of steam temperature at the outlet of high-temperature reheater at low load.