Abstract: Sargassum fusiforme is rich in a variety of nutrients and active substances, and is one of the important economic seaweeds in China. Its quality could be easily affected by environmental factors such as marine areas. To figure out the differences in nutritional composition of Sargassum fusiforme cultivated in different marine areas, this paper used Sargassum fusiforme cultivated in seven marine areas in Zhejiang Province as the research object, and determined the basic nutrients, amino acids and fatty acids using the National Food Safety Standard. The antioxidant activities of the seven Sargassum fusiforme polysaccharides were determined using DPPH· and ·OH scavenging as evaluation criteria, and the structural composition of seven kinds of Sargassum fusiforme polysaccharides were preliminarily analyzed. The results showed that, the moisture content of seven Sargassum fusiforme was highest, between (82.50±0.32) and (86.50±0.23) g/100 g. In addition to moisture content, the other main nutrient component was polysaccharide. The polysaccharide content of Sargassum fusiforme cultivated in No.7 marine area was the highest (P<0.05), which was 2.51 g/100 g. The fat content of Sargassum fusiforme of each marine area was relatively low, which was (0.30±0.01)~(0.40±0.01) g/100 g, indicating that Sargassum fusiforme was a kind of typically low-fat food. The amino acid content of Sargassum fusiforme varied greatly and the EAAI (Essential Amino Acid Index) values of Sargassum fusiforme in each marine area were all more than 0.9, indicating that Sargassum fusiforme in the seven marine areas were all high-quality protein sources. Furthermore, the amino acid composition ratios of Sargassum fusiforme cultivated in No.2 and No.6 marine areas were reasonable, which were closer to the ideal protein model recommended by WHO/FAO in 1937. A total of eight fatty acids were detected in Sargassum fusiforme from seven marine areas. Except for Sargassum fusiforme cultivated in No.2 marine area, the rest were mainly polyunsaturated fatty acids. The heavy metal content of Sargassum fusiforme complied with the requirements of in National food safety standard-Maximum levels of contaminants in foods (GB 2762-2017). The contents of Ca and K in Sargassum fusiforme cultivated in No.4 marine area were (131.44±6.36) mg/100 g and (1687.63±28.14) mg/100 g, respectively, which were good sources of Ca and K. The polysaccharide structures of Sargassum fusiforme in each marine area were similar, all of which contained sulfate groups and had the activity of scavenging ·OH and DPPH·. Among them, the polysaccharide of Sargassum fusiforme cultivated in No.5 marine area had the highest antioxidant activity, and the IC₅₀ of scavenging ·OH and DPPH· were 0.534 and 0.236 mg/mL, respectively. This study laid a certain foundation for the scientific breeding, processing and quality grade evaluation of Sargassum fusiforme.