Abstract:To investigate the effect of extraction methods on the spatial structure and biological activity of polysaccharides from Laminaria japonica （LJP），three types of LJP were obtained from Laminaria japonica dry powder by hot water，acid-assisted，and base-assisted extraction. High performance liquid chromatography（HPLC），Congo red reagent，and other methods were used to study the spatial structure characteristics of LJP，explore the correlation between the spatial structure and rheological properties of LJP，and establish an oxidative damage model of cells to further investigate the protective effect of LJP on oxidative damage of cells. The results showed that LJP by base-assisted （BLJP） extraction had a high yield [（9.52±0.05%）]，while LJP by acid-assisted （ALJP） extraction had the highest content of glucuronic acid [（37.56±0.88%）]. LJP by hot water（HLJP） extraction had the highest content of sulfate groups [（15.96±0.44%）]；their weight average molecular weights were 1 547.94 kDa（HLJP），104.10 kDa（ALJP），and 2 547.65 kDa（BLJP），respectively；the shear rheological curve indicated that the LJP solution was a non-Newtonian fluid with shear thinning phenomenon. Power-Law linear fitting showed that the highest viscosity coefficient of BLJP was 1.275 1 Pa·sn，and the lowest viscosity coefficient of ALJP was 0.207 9 Pa·sn，corresponding to its weight average molecular weight；At 5 mg/mL，the highest hydroxyl radical scavenging activity of ALJP was 81.08%，which could reduce the secretion of malondialdehyde （MDA） in RAW264.7 cells damaged by H2O2 oxidation from 2.53 nmol/mL to 1.57 nmol/mL.