In the present study, the antioxidant activities and immunostimulatory ability of a polysaccharide extracted from Chinese Sesbania cannabina, which was identified to be a galactomannan in our previous study, were investigated. The extracted polysaccharide exhibited strong DPPH, ABTS and hydroxyl radical scavenging activities and ferrous ion chelating activity in a concentration-dependent manner. The immune-enhancing effect of our polysaccharide on RAW 264.7 macrophage cells was investigated by determining the cell viability, phagocytic activity, NO and intracellular ROS production and mRNA expression of cytokines. The results indicated that the polysaccharide could increase the production of NO and intracellular ROS, as well as effectively trigger transcriptional activation of TLR-2/4, NF-κB, IL-10/1β/6, IFN-γ, Ik-Bα, iNOS, COX-2 and TNF-α. These findings provide useful information for potential application of the polysaccharide extracted from Chinese Sesbania cannabina in the food industry.