Abstract: Astragalus polysaccharides are the most immunoregulatory active and abundant substances in Astragalus, with anti-tumor, anti-viral, and immune-promoting biological activities. They have been widely used in clinical practice. Previous studies have found that Astragalus polysaccharides are mainly composed of two different polysaccharides, APS-Ⅰ (> 2 000 kDa) and APS-Ⅱ (10 kDa), with APS-Ⅱ (10 kDa) being the most active component of Astragalus polysaccharides. We used α-1, 4-glucan endo-hydrolysis enzyme to degrade APS-Ⅱ into oligosaccharides, and screened the immune activity of oligosaccharides in vitro. We found that the overall immune activity of 2-9 oligosaccharides was low, while the immune activity of 10-14 oligosaccharides was strong, and the activity was better than that of untreated APS-Ⅱ. To investigate the key structural features of APS-Ⅱ oligosaccharides that play a role in immune activity, we used MALDI-TOF-MS biochemical mass spectrometry and high-resolution mass spectrometry instrument ESI-Q Exactive-MS to analyze the APS enzymatic oligosaccharides. By comparing, we found that 10-14 oligosaccharides contain 1→4 and 1→6 branched structures with coexisting linkages, suggesting that 1→4 and 1→6 linkages in branched structures are key structural features that play a role in the immune activity of APS-Ⅱ, laying a theoretical foundation for the structure-activity relationship of Astragalus polysaccharides and oligosaccharides.