The polysaccharides were obtained by microwave-assisted enzymatic extraction from Porphyra, and then modified by sulphate method. Single factor and orthogonal experiments were used to optimize the sulfated modification process of polysaccharides, which were identified by Fourier transform infrared spectroscopy (FT-IR). The antioxidant and hypoglycemic activity changes of the polysaccharides modified by sulphate and chlorosulfonic acid-pyridine method were also studied. Results showed that the optimal technology parameters were the ratio of solid to liquid 1:80 g/mL, the mass ratio of Porphyra polysaccharide to ammonium sulfate 10:9 g/g, reaction time 33 min, and the maximum degree of substitution was 2.94. FT-IR showed that the characteristic absorption peaks of sulfate radical group appeared near 801 and 1123 cm−1, and the possible substitution position of sulfate radical group was C-6. The sulfate modification could significantly improve the scavenging ability of DPPH, O2− and OH free radicals and the inhibitory effect on α-glucosidase activity of the polysaccharides (P
