In this paper, the flow behavior of gas−solid flow in the standpipe of a circulating fluidized bed (CFB) and its influence on the system pressure balance and hydrodynamic performance have been studied. A CFB apparatus with a square riser of 0.1 × 0.1 m in cross-section and 4.5 m in height and a standpipe of 3.0 m in height and 0.08 m in diameter were established. The standpipe connected the riser to a loop seal on one end and connected to the cyclone on the other end. The experimental results showed that the flow behavior in the standpipe is not only strongly related to the particle holdup, the structure, and the aeration flow rate of the standpipe and loop seal but also strongly coupled with the pressure drops of the rest of the parts of the CFB system. At fixed-bed inventory Mt and fluidizing gas velocity in the riser Ug, the solid circulating rate GS increases with the flow rate of aeration air Q in the loop seal. With an increasing GS, the pressure drop of the loop seal decreases, while both the press...