The invention relates to a preparation method and application of a water product bacteriostatic agent. The method is characterized by comprising the following steps: dissolving a proper amount of carrageenan oligosaccharide in deionized water; adjusting the pH value to 5.5-6.5; adding a mixture of ferrous chloride and zinc chloride in the mass ratio of 1:2-4, wherein the weight ratio of carrageenan oligosaccharide to the mixture of ferrous chloride and zinc chloride is 5-8:1; meanwhile, adding an appropriate amount of Vc for protection; reacting at the temperature of 25-30 DEG C for 2-3 h; after the reaction, conducting ethanol precipitation and centrifugation to obtain a chitosan-metal derivatives sediment; freeze-drying to obtain a carrageenan oligosaccharide-metal derivative; mixing the prepared carrageenan oligosaccharide-metal derivative with carrageenan oligosaccharide by the ratio of 1-2.5:1 to obtain the water product bacteriostatic agent. The preparation process is simple; and the prepared antibacterial agent has obvious antibacterial effect, can effectively conduct antibacterial protection on frozen aquatic products and is easy to clean for consumption, and free of residual; and the antibacterial agent can effectively guarantee the food safety of aquatic products while fresh-keeping.