Abstract: Experimental studies were carried out in a specially designed high-density “riser-downer-riser” coupled circulating fluidized bed system. Fluidized catalytic cracking (FCC) particles were used.Compared with the primary air rate, the secondary air rate had an important effect on the solids circulation flux. When the solids circulation flux was 400 kg•m^-2•s^-1, the apparent solids holdup exceeded 20% near the top of the first riser, and the volumetric solids fraction (apparent solids holdup) was larger than 5.5% in the fully developed region of the downer.With increasing solids circulation flux, more uniform solids concentration distribution along radial direction could be obtained.