This paper first provides that Chinese yam polysaccharide (CYP) is encapsulated by PLGA using a double emulsion solvent evaporation method and aims to screen the optimal preparation of CYP-PLGA nanoparticles (CYPP) using response surface methodology (RSM). The volume ratio of the internal water phase to the organic phase (W1:O), the volume ratio of the primary emulsion to the external water phase (PE:W2) and the concentration of Poloxamer 188 (F68) are deemed key variables for the encapsulation efficiency of CYPP. The results demonstrated that the data were accurately fitted into the RSM model. According to the RSM, the optimal scheme was a volume ratio of W1:O of 1:9, a volume ratio of PE: W2 of 1:10 and a concentration of F68 (W/V) of 0.7%. TEM and SEM images demonstrated that the nanoparticles had a spherical shape and smooth surface. The CYP and CYPP in vitro release studies demonstrated that the CYPP showed a release rate 53.41% lower than the release rate of CYP after 48 h. The result of pro-proliferation and flow cytometry emerged that the CYPP were more effective compared with the free CYP and blank PLGA nanoparticles in promoting lymphocyte proliferation and triggering the transformation of T lymphocytes into Th cells.