Objective To investigate the effect of exosomes derived from osteosarcoma stem cells (OSCs) on the proliferation and differentiation of T cells and to explore its mechanism. Methods OSCs were obtained using serum-free suspension culture and identified using stem cell marker logistics separation technology. The exosomes (OSCs-exo) secreted by OSCs were extracted using an exosome extraction kit and identified using transmission electron microscopy ( TEM), particle size analysis, and Western blot. Coculture of OSCs-exo with peripheral blood mononuclear cells, carboxyfluorescein succinimidyl ester (CFSE) staining, and flow cytometry were used to detect the effect on T-cell proliferation. Western blot was used to detect the phosphorylation of extracellular signal-regulated kinase ( ERK) protein in the ERK signaling pathway. Immunomagnetic beads were used to separate CD4⁺ T cells and coculture them with OSCs-exo in the differentiation medium induced by different CD4⁺ T cell subsets. Western blot was used to detect the expression of phosphorylated STAT1, STAT3, and STAT5 proteins in the STAT signaling pathway. Results OSCs CD133⁺MG-63 was successfully isolated. The result of TEM, particle size analysis, and Western blot showed that OSCs-exo is a round or oval vesicle with a diameter of 30-100 nm. The CFSE staining, flow cytometry, and Western blot result showed that OSCs-exo inhibits the proliferation of CD3⁺ , CD4⁺ , and CD8⁺ T cells by inhibiting the phosphorylation of ERK protein, and inhibits the differentiation of CD4⁺ T cells to Th1, Th17, TH₂, and Treg cells by inhibiting the expression of phosphorylated STAT1 and STAT3 protein. Conclusions Exosomes derived from OSCs can inhibit the proliferation of T cells by downregulating the phosphorylation of ERK protein and reducing the expression of phosphorylated STAT1 and STAT3 protein to induce CD4⁺ T cells to differentiate into TH₂ and Treg cells. They can also inhibit the differentiation of CD4⁺ T cells into Th1 and Th17 cells to downregulate cellular immune function and promote tumor progress.