Severe pressure fluctuations in parallel external loops were found in the circulating fluidized bed (CFB) boilers recently, so a CFB test rig with two external loops was established to investigate the unstable performance of parallel external loops. A long-length horizontal section was used in the loopseal to intensify the impact of gas flow inside horizontal section on solid circulation rate, and CO2 tracer method was used to detect the variation of gas flow rate in the horizontal section. The results showed that the solid circulation rate would significantly influence the performance of parallel external loops. The pressure drop in the parallel external loops at low solid circulation rate was relatively stable if aeration air distributed evenly between the two parallel external loops, and the high-frequency pressure fluctuation in the riser was ascribed to the bubble behavior at turbulent fluidization. However, the fluctuation of gas flow rate inside the horizontal section aggravated at high solid circulation rate, leading to the unstable solid recycling rate, as well as the low-frequency and high-amplitude fluctuation of pressure drop in the parallel external loops. For this reason, a single blower is not recommended to use to supply the aeration rate among parallel external loops. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.