The invention relates to a stress-resistant protective agent for saccharomyces cerevisiae, in particular to a stress-resistant protective agent based on alginate oligosaccharides as well as a preparation method and an application of the stress-resistant protective agent. The stress-resistant protective agent mainly comprises the alginate oligosaccharides and a solvent, wherein the mass concentration of the alginate oligosaccharides is 1 mu g/mL-10mg/mL; the alginate oligosaccharides comprise mannuronate, guluronic acid or a hybrid of mannuronate and guluronic acid, the average degree of polymerization is 5-8, and the average molecular weight ranges from 2,000 to 3,000. The stress-resistant protective agent is added to a culture medium of saccharomyces cerevisiae, the application field of marine-derived polysaccharides and oligosaccharides is expanded, the high-glucose fermentation time is shortened, the fermentation strength is improved, and the survival rate and the cell activity of saccharomyces cerevisiae in multiple stress environments are increased.