Response surface methodology (RSM) combined with a Box-Behnken design (BBD) was used to optimize the hot-water extraction (HWE) conditions which were determined to be: liquid-solid ratio 29.25:1 mL/g, extraction time 1.66 h, extraction temperature 95 °C, the optimized yield of polysaccharides 0.47 ± 0.01% (n = 3). Hot-water extraction polysaccharides (HWEP) and Microwave-assisted extraction polysaccharides (MAEP) both consist of Rha:Xyl:Ara:Fru:Glu with the molar ratio of 1.05:1.21:3.86:1:3.61:4.5 and 1:1.95:1.72:1.78:4.36:6.18, respectively. Ultrasound-assisted extraction polysaccharides (UAEP) consists of Rha:Xyl:Ara:Fru:Man:Glu with the molar ratio of 1:2.31:5.23:1.05:3.17:4.17:7.89. The molecular weight distribution of HWEP, MAEP and UAEP ranged from 16 kDa to 1698 kDa, 15 kDa to 913 kDa, and 17 kDa to 1118 kDa, respectively. The absorption peaks in FT-IR confirmed the skeletal modes of the pyranose ring in polysaccharides. The second derivative of FT-IR proved difference of polysaccharides obtained from different extraction methods. The antioxidant activity investigations shown all three polysaccharides extracts possess high scavenging activity of DPPH radicals, hydroxyl radical and ABTS+ radical. Polysaccharides from Carex meyeriana Kunth (CMKP) might be potentially used for various practical applications such as medical and food industries, and this paper provides a theoretical basis and reference for further study of CMK.