The invention discloses a fresh-keeping synergist for fresh marine functional spirulina. 100ml of fresh-keeping synergist is prepared by mixing 0.5-3g of marine sulfated galactooligosaccharides (oligomerization carrageenan), 0.5-5g of isomaltose hypgather, 0.1-1g of citric acid, 0.1-1g of Vitamin C, one or multiple of 0.1-0.3g of carboxymethylcellulose, 0.1-0.4g of agar, 0.1-0.3g of sodium hyaluronate and 0.1-0.3g of bacterial cellulose, and the balance of purified water. A specific preparation method comprises the steps of: adding the marine sulfated galactooligosaccharides, the isomaltose hypgather, the citric acid, and one or multiple of the carboxymethylcellulose, the agar, the sodium hyaluronate and the bacterial cellulose into the purified water according to quantity in a formula, then heating for boiling for 20 minutes, standing for cooling, adding the Vitamin C, and mixing to reach a uniform state to obtain the fresh-keeping synergist. When in use, the fresh-keeping synergist provided by the invention is mixed with the fresh spirulina according to the proportion of (1:10)-(3:10). The fresh-keeping synergist not only plays a role in keeping freshness for the cultivated fresh spirulina, but also has the functions of sterilizing, resisting to virus, oxidation and tumor, enhancing immunity, reducing blood sugar and blood fat, promoting growth of intestinal probiotics, improving cardio-cerebral vessels and the like, so that the effect of increasing the spirulina is enhanced.