Reed straw is an abundant biomass resource throughout the world. Arabinoxylan is the main component of the hemicellulose in reed straw. A probe into the structure and rheological behavior of arabinoxylan from reed straw (AXRS) is valuable for its effective utilization. In this study, AXRS was extracted using 20% tetramethylammonium hydroxide (TMAH) aqueous solution as a solvent. AXRS consisted mainly of D-xylose and L-arabinose with a low molecular weight (104 kDa) according to HPLC and GPC analysis. FT-IR and NMR analyses showed AXRS had similar structure to arabinoxylans from other plants, which was further verified by XRD. TG analysis displayed that water loss from AXRS occurred in the temperature range of 30 °C to 240 °C, while degradation mostly took place between 240 °C and 350 °C under both air and nitrogen atmospheric conditions. The rheological measurements indicated that AXRS was completely dissolved in 20% TMAH aqueous solution and aggregated after reaching the critical concentration of around 0.062 g mL−1. With the enhancement of AXRS concentration, the storage modulus gradually increased, suggesting that AXRS aggregates may form in 20% TMAH aqueous solution. Display omitted •The characterization of arabinoxylan from reed straw (AXRS) shows that its main components are D-xylose and L-arabinose.•Rheological behavior of AXRS in 20% TMAH aqueous solution shows that its critical concentration is about 0.062 g mL−1.•This study is helpful to understand the properties of AXRS and boost its applications.