Extraction yield of polysaccharides from Achyranthes bidentata Radix (AbR) was increased by steam explosion pretreatment. Based on the results of single-factor experiments, the response surface methodology was employed to further optimize the steam explosion pretreatment conditions. The effect of steam explosion pretreatment on the DPPH radical scavenging activity and O^2- radical scavenging activity of Achyranthes bidentata Polysaccharides (AbPS) was evaluated. The optimum conditions was determined as follows: the water/AbR ratio 11%, steam explosion pressure, 1.6 MPa; residence time 63 s. After pretreatment under the optimum conditions, the extraction yield of polysaccharides from AbR was found to be 11.88%, which was twice as much as that from the untreated AbR．It maybe caused by the physical damage on the surface of AbR, including partial rupture with deep longitudinal cracks and micro-holes, due to the pretreatment of steam explosion, which can be seen from the SEM micro photographs. Furthermore, the antioxidant activity of AbPS was enhanced strongly after steam explosion pretreatment. The DPPH radical scavenging activity of AbPS increased from 77.9% to 90.1%, while the O^2- radical scavenging activity of AbPS was around 1.3 times as much as that from untreated AbR.