The invention relates to a method for producing sea tangle nutrient-regulating feed additive by using marine bacteria. The method comprises the following steps: pre-treating sea tangle as fermentation raw material by washing, over-drying, powdering and sterilization to obtain sea tangle powder, mixing the sea tangle powder with 1-8% NaCl solution at a sea tangle powder/NaCl solution weight ratio of 1:(10-80) to obtain fermentation substrate, inoculating alginic acid-decomposing bacteria to the fermentation substrate in an inoculation amount of 2-10%, and allowing fermentation under such conditions that the fermentation temperature is in the range of 15-25 DEG C, the pH value is in the range of 6.0-8.0, the fermentation rotation speed is 80-140 r/min and the fermentation time lasts 24-72 hours. The fermentation product is the sea tangle nutrient-regulating feed additive. The method retains the nutritional value of sea tangle, and increases the contents of protein and active oligosaccharide. The product is used as nutrient-regulating additive for animal feeds to increase the nutritional value of feed and provides disease-preventing, immunostimulant and growth-promoting effects.