The study is aimed at investigating the immunoenhancement activity of polysaccharides from Platycodon grandiflorum polysaccharides (PGPSs) in vitro. In this study, some research on lymphocyte proliferation, cell cycle, and the levels of CD4⁺ and CD8⁺ T cells were performed. Four different concentrations of PGPSs (PGPS_tc, PGPS_60c, PGPS_80c, and PGPS_tp) were harvested and added to peripheral blood T lymphocytes. We observed significant increases in T lymphocyte proliferation at PGPS_tc groups individually or synergistically with phytohemagglutinin (PHA) at most concentrations, and their lymphocyte proliferation rates were the highest. The active sites of PGPS_tc and PGPS_60c were subsequently chosen. Then, we utilized flow cytometry to determine lymphocyte cell cycle distribution and levels of CD4⁺ and CD8⁺ T cells. At most time points, PGPS_tc could facilitate lymphocyte cell cycle progression from the G0/G1 phase to the S and G2/M phases and, simultaneously, increase the levels of CD4⁺ and CD8⁺ T cells. These results indicate that PGPS_tc enhances the immune functions, suggesting that PGPS_tc could be a potential immunopotentiator for further in vivo and clinical trial experiments.