Abstract: The once-through boiler unit faces the control problem of switching between dry and wet states during flexible operation in ultra-low load range. The difference in water-steam flow process between dry and wet states operation results in different control object structures, so it is necessary to establish a dynamic model suitable for both dry and wet states; the different control objectives during dry and wet states operation result in different structures of the basic control methods. Therefore, a 2-layer flexible coordinated control system is proposed to address the characteristics of changes in object structure, control structure, and control objectives during the conversion of dry and wet states. The existing basic control methods of dry and wet states operation are used in the bottom layer of flexible control, and the optimization algorithm is used in the top layer to calculate the flexible factor in real time. The key of the algorithm is to provide response matrices for different stages of conversion, and optimize them in segments based on the flexible factor. Simulation experiments show that the flexible coordinated control method can achieve automatic conversion between dry and wet states of the system, and the controlled parameters meet the control requirements.