Abstract: Due to its compactness, high efficiency and flexibility, the potential applications of advanced supercritical carbon dioxide (S-CO₂) Brayton cycle in Generation Ⅲ of Solar-thermal systems and Generation Ⅳ nuclear reactors are fantastic. The flow instability in the Brayton cycle system threatens the safe operation of the unit. To study the flow instability of CO₂ in parallel vertical upward channels, an experimental system is established, and the effect of operational parameters on the flow instability is obtained. A prediction model on CO₂ flow instability is established by introducing a dimensionless throttling coefficient. The predicted value is in good agreement with the experimental value (±20%). It is found that increasing the system pressure and the mass flow, and reducing the subcooling (ΔT_sub=5~20 ℃) can increase the flow stability between parallel tubes. With the increase of outlet throttling, the flow stability between parallel tubes decreases, and the stability boundary of heat load gradually decreases.