CM-LAP, P-LAP were prepared by chloroacetic acid method and sodium phosphate method, respectively. The viscosity, solubility, molecular weight, monosaccharide composition, and morphology were determined to explore their physicochemical properties, with verification by infrared and nuclear magnetic resonance. In vitro antioxidant experiments and evaluation of the protective effects against oxidative damage induced by H2O2 on HepG2 cells were conducted to assess the antioxidant activity of LAP, CM-LAP, and P-LAP. The results show that the modified polysaccharides showed reduced viscosity, increased solubility, unchanged monosaccharide composition but significant changes in monosaccharide molar ratios. Both crystalline and amorphous structures were observed for LAP, CM-LAP, and P-LAP, with alterations in surface morphology after modification, while the triple helix structure remained unchanged. Thermal stability was enhanced, with residual carbon rates at 550 ℃ recorded as 22.76%, 40.90%, and 30.38% for LAP, CM-LAP, and P-LAP, respectively. In addition, infrared and nuclear magnetic resonance spectroscopy confirmed the successful preparation of CM-LAP and P-LAP. The evaluation of their in vitro antioxidant activity showed that the modified polysaccharides enhanced LAP ability to DPPH and -OH scavenge free radicals. LAP, CM-LAP, and P-LAP were able to reduce the MDA levels induced by H2O2, with MDA levels decreasing from 6.768 nmol/mg protein to 4.029, 3.517, and 3.772 nmol/mg protein, respectively. Furthermore, they increased the activities of SOD and GSH-Px, with SOD activities increasing from 6.086 U/mg protein to 6.991, 7.474, and 7.192 U/mg protein, and GSH-Px activities increasing from 7.019 U/mg protein to 8.017, 8.591, and 8.227 U/mg protein. These results demonstrate that carboxymethylation and phosphorylation modifications can enhance the physicochemical properties and antioxidant activity of Lavandula angustifolia polysaccharides, highlighting their potential applications.