Adjuvants as essential ingredients amplify the magnitude and durability of immune responses in various vaccine strategies. Polysaccharides with potent immunoenhancing effects are widely applied as promising vaccine adjuvants, however, they have rarely been licensed for use in human vaccines due to the limitation of their efficacy and safety. Moreover, nanoparticles not only act as antigen drug delivery vectors but also possess intrinsic adjuvant functions, revealing the dual effects of nanoparticles in augmenting antigen-specific immune responses. Intriguingly, nanoparticle forms can enhance the immunostimulatory potency of polysaccharide adjuvants, since polysaccharide nanoparticles exert more excellent adjuvant effects than polysaccharides in initiating humoral, cellular and mucosal immune responses. Emerging evidence has also suggested that multiple immune-related signaling pathways including cGAS-STING, NLRP3, TLRs, cell death or metabolism signaling probably participate in the immunomodulation of polysaccharide nanoparticles, but systemic investigations into the adjuvant mechanism are still inadequate. This review aims to give an updated summary and discussion on the adjuvant function and mechanism of polysaccharide nanoparticles for understanding their superior adjuvant property and effectively utilizing them as potent immune adjuvants in vaccine development.