In this study, BCPS-1, a polysaccharide previously isolated and characterized from Bupleurum chinense was chemically modified to yield two sulfated derivatives, which we designated as S-BCP_1-4 and S-BCP_1-8. The physicochemical properties of these sulfated derivatives were then determined by high performance liquid chromatography (HPLC), Fourier transform infrared spectrometry (FT-IR), and gas chromatography (GC), and then compared with those of BCPS-1. Furthermore, the antioxidant activities of all three polysaccharides were also evaluated using 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical assay, superoxide radical assay and hydroxyl radical assay, while their effects against H₂O₂-induced cellular senescence were determined using senescence-associated β-galactosidase (SA-β-gal) staining, cell cycle assay and immunoblotting in H₂O₂-induced mouse lung endothelial cells (MLECs). Compared to BCPS-1, S-BCP_1-4 and S-BCP_1-8 exhibited remarkable antioxidant effect, and in a concentration-dependent manner. They also provided stronger protection against H₂O₂-induced cellular senescence in MLECs. These results indicated that the sulfate group in the modified B. chinense polysaccharides might play an important role in radical scavenging and resistance to H₂O₂-induced senescence. These sulfated polysaccharides could be considered as novel pharmaceutical products with potential antioxidant and anti-senescence effects.