In this paper, the design principle of Fluidization State Specification (FSS) is reviewed and its applications in CFB boiler are introduced. In the early 2000s, FSS design principle was proposed to determine the fluidization state in the upper furnace as the basis of engineering design. The principle was initially adopted by engineers to select a proper superficial gas velocity to prevent the severe erosion on water walls. It was used in many CFB furnace designs including the world's largest 600 MW supercritical one in China. Later, FSS design principle was extended to reduce the pressure head of the draft fan to save energy consumption. Industrial application showed that by changing the amount of bed inventory, CFB boilers could operate at a re-constructed fast bed state, not only saving -30% power of the draft fans, but also further reducing the erosion on water walls and improving combustion efficiency. Recently, FSS design principle was further improved and applied to achieve ultra-low NOx and SO2 emission. A third coordinate with improved bed quality, i.e., size distribution of bed material was proposed to add on the original two-dimensional fluidization state diagram defined by the superficial gas velocity and the solid circulation rate. Industrial practices showed that under the guideline of the improved FSS design principle, it was possible for CFB boilers to reach over 99% desulfurization efficiency with limestone injected into furnace, and less than 50 mg/Nm(3) (at 6% O-2) NOx emission at the furnace exit. FSS design principle promotes the booming of ultra-low emission CFB boilers in China.