Objective: This study aimed to investigate the immunomodulatory effects of total polysaccharides from Sijunzi Decoction and elucidate their potential mechanisms， while comparing the differences in immunomodulatory activity between Sijunzi Decoction polysaccharides and polysaccharides from its individual constituents (Panax ginseng， Poria cocos， Atractylodes macrocephala Koidz， Glycyrrhiza uralensis Fisch). The research sought to interpret the contribution characteristics of functional components in classical formulas from the perspective of food science and nutrition， explore key bioactive substances in classical formulas， and provide theoretical support for the scientific interpretation of traditional prescriptions' functional characteristics and their applications in functional foods. Methods: An immunosuppressed mouse model was established using cyclophosphamide. The immunomodulatory effects of Sijunzi Decoction polysaccharides and individual polysaccharides were systematically evaluated based on body weight changes， immune organ indices， histopathology， and levels of immunoglobulins (IgA， IgM， IgG) and cytokines. Results: Sijunzi Decoction polysaccharides significantly alleviated weight loss in immunosuppressed mice， restored spleen and thymus indices， and enhanced serum IgA， IgM， IgG levels and TNF-α secretion， with superior effects compared to individual polysaccharides. Furthermore， Sijunzi Decoction polysaccharides improved metabolic disorders by regulating key pathways such as α-linolenic acid metabolism and arachidonic acid metabolism. Among individual polysaccharides， ginseng polysaccharides exhibited the most pronounced effects， affirming their role as the 'monarch drug' in the formula. Conclusion: Sijunzi Decoction polysaccharides serves as the primary active component responsible for the immunomodulatory effects of Sijunzi Decoction， mediated through the regulation of critical metabolic pathways (e.g.， α-linolenic acid metabolism) and enhancement of both humoral and cellular immune functions. Ginseng polysaccharides were identified as the major contributing component within Sijunzi Decoction polysaccharides. This study offers novel insights into the utilization of medicinal-edible homologous polysaccharides and their mechanisms， though future research integrating transcriptomics and proteomics is required to validate molecular targets.