A water-soluble polysaccharide extracted from Gastrodia elata was chemically modified to generate acetylated and carboxymethylated derivatives. Thourshe conditions for the two modification methods were optimized through single factor and response surface analyses. The optimized conditions for acetylation were 3.4 mL of acetic anhydride, a reaction temperature of 63.4 °C, and a reaction duration of 4 h. For carboxymethylation, the optimal modification parameters were 2.2 g of chloroacetic acid, a temperature of 62.3 °C and a NaOH concentration of 2.2 mol/L. The results of chemical analysis and Fourier transform infrared spectroscopy revealed that these modifications were successfully added, although the molecular weights of the derivatives decreased because of slight degradation during the reactions. The antioxidant and immunomodulatory potentials of the polysaccharide derivatives were assayed to delineate the structure-bioactivity correlations. The results revealed that both acetylation and carboxymethylation of GEP-1 augmented its scavenging efficacy against DPPH radicals, hydroxyl radicals, and superoxide anions, surpassing the antioxidant capacity of the unmodified polysaccharide. Furthermore, the modified derivatives demonstrated minimal cytotoxicity against RAW264.7 macrophages and significantly potentiated the secretion of TNF-α and IL-6 by these cells, indicative of a pronounced immunostimulatory response. These findings suggest that chemical modifications can significantly improve the solubility, antioxidant capacity, and immunomodulatory activity of polysaccharides, thereby enhancing their potential for development and application in the utilization of natural polysaccharide resources.