Abstract: The optimum condition of extraction of polysaccharides by ultrasonic-associated enzyme method and the antimicrobial activity of the polysaccharides as well as their chemical modified products to common foodborne pathogens were studied to provide information to application of polysaccharides in food additives and biological control. Single-factor and orthogonal experiments were used to determined the optimum conditions of polysaccharides extraction by ultrasonic-associated cellulase and neutral proteinase method. The oxidation depolymerization and acetylation were acquired by chemical modified polysaccharides. The antibacterial activity of polysaccharides, oxidation depolymerization and acetylation to 8 different foodborne pathogens, including 3 kinds of molds and 5 kinds of bacteria, were determined by disk-diffusion method. The optimum parameters of ultrasonic-associated enzyme method were pH at 4.5, extracting temperature at 50℃, enzyme compounds including cellulose and neutral proteinase with a ratio of 2 : 1 value at 2.25% accompanied to extract with 75 min, under this condition, polysaccharides extraction rate was up to 8.64%. Laminaria japonica polysaccharides can inhibit the growth of Escherichia coli, Salmonella, Staphylococcus aureus and Enterococcus faecalis in a certain extent, but have no inhibitory effects on Shigella. The oxidation depolymerization and acetylated fucoidan of Laminaria japonica polysaccharides show more significant inhibitory activity against those foodborne pathogens than Laminaria japonica polysaccharides, and has inhibitory effect on Shigella. However, three polysaccharides have no inhibitory activity on 3 experimental molds, namely Penicillium, Rhizopus and Aspergillus niger.