The invention discloses a biological method for preparing rachycentron canadum expanded compound feed by salicornia. The shortcoming of high fishmeal consumption of existing feed is overcome, waste materials are changed into wealth, and salicornia resources are developed and used in a high value manner. The feed mainly comprises salicornia fermentation powder, steam fish meal, fermented soybean meal, double-low rapeseed dregs, DDGS (distillers dried grains with soluble), flour, sodium alginate tech grade, sea fish oil, corn germ oil, soybean lecithin, squid ointment, choline chloride, glycine betaine, inositol, compound vitamins, compound microelements, allicin, bacillus licheniformis, bifidobacterium bifidum, phytase, non-starch polysaccharide enzyme and the like, and the salicornia fermentation powder comprises, by weight, 30 parts of yam flour, mongolian milkvetch roots, licorice roots, dried ginger, malt, bran, sugar stevia leaves and the like which prepare the salicornia fermentation powder by the aid of a two-step fermentation process. The feed has the advantages that rapid growth of rachycentron canadum is remarkably facilitated, aquaculture survival rate is increased, aquaculture cost is reduced, aquaculture digestion and absorption rate is increased, nitrogen and phosphorus discharge is reduced, aquaculture environments are improved, and the feed has high cost performance and popularization and application values.