Abstract: Grateloupia C. Agardh, a type of large rhodophyta, is widely distributed in China’s shallow sea, and its polysaccharides have excellent biological activities. This article focused on the research of biological activities of Grateloupia C. Agardh polysaccharides to develop a new and more efficient probiotics, which can improve the body’s immunity and enhance physical fitness, or become a growth-promoting factor of bifidobacteria in the fermentation industry of yogurt. Experiments used 96-well plate method to study probiotics growth of Grateloupia C. Agardh polysaccharides, which extracted by water extraction, the oligofructose group as a control. In the biological activities in vitro test, the research showed that the biological activities of the polysaccharides was better than oligosaccharide; in the bacteriostatic effect test, it showed that Grateloupia C. Agardh polysaccharides and oligosaccharide had no effect in the growth of Escherichia coli and Staphylococcus aureus, instead, they all had weaker promoting effect, but their negative effects were within the acceptable range of the body. It is preliminarily judged that the polysaccharide had good biological activities and can be studied further as a prebiotics.