Achyranthes bidentata is an important Traditional Chinese Medicine for the treatment of osteoporosis. In this study, A. bidentata polysaccharide (ABPB), which was extracted with alkali from the root of A. bidentata at room temperature, significantly increased the bone mineral density, bone mineral content, trabecular thickness, trabecular number and biomechanical properties of ovariectomized (OVX) rats, indicating that ABPB had prominent curative effects on osteoporosis in OVX rats. A novel polysaccharide (ABPB-3) was purified from ABPB, and its structure was characterized as a repeating unit consisting of →4)-α-d-GalpA-(1→, →2,4)-α-l-Rhap-(1→, →5)-α-l-Araf-(1→, →2,3,5)-α-l-Araf-(1→, →3)-β-d-Galp-(1→, →3,4,6)-β-d-Galp-(1→, terminated with α-l-Araf, α-l-Rhap and β-d-Galp. Up to now, there were no literature reports relevant to the structure of ABPB-3. In the zebrafish model of glucocorticoid-induced osteoporosis (GIOP), ABPB-3 significantly increased the relative fluorescence intensity of the skull bone mass in a concentration-dependent manner, indicating that it stimulated bone formation activity. Thus, ABPB and ABPB-3 have the potential to be used for the anti-osteoporosis medicine.