To promote the implementation of the “double carbon” policy and to manage the high volatility of new energy, the current requirements for deep peak shaving of thermal power units are getting higher and higher. Circulating fluidized bed (CFB) boiler units have inherent advantages under low load operation conditions. However, the ultra-low load operation below 20% still faces many difficulties, such as the stability of the fluidization in the furnace, the emission of nitrogen oxides, and the safety issues caused by local over-temperature in the furnace. This paper took the application of deep peaking technology of a 330 MW CFB boiler as an example and introduced the modification of the unit components, such as the coal conveying screening and crushing system, throttling rings of wind caps and lower secondary air pipes, etc. With the application of flue gas recirculation and other technologies, an 18% ultra-low load operation was successfully realized on this CFB boiler, and the NOx emission was also well controlled. The key points and difficulties of the ultra-low load operating technology of CFB unit were summarized. The potential problems caused by deep peaking operation were analyzed, and corresponding measures were proposed. The results of this study have an important engineering reference value.