The present invention provides algin oligosaccharides having polymerized degree of 2-22 and the derivatives thereof, such algin oligosaccharides are made of mannuronic acid bonded by ±-1,4 glycosidic bond. The derivatives which the reduced terminal in position-1 is carboxyl radical can be obtained by oxidation. The invention also provides the manufacture of algin oligosaccharides and the derivatives thereof, which includes the alginate solution in water reacts in the reactor under the condition of high press, 100-120°C, pH2-6, for 2-6 hours, after the reaction completed, its pH value was adjusted to 7. The result oligosaccharides are oxidated in presence of the oxidant, and then produce the oxidated products. The algin oligosaccharides and the derivatives thereof of the invention can be used in the manufacture of medicaments for preventing Alzheimer's dementia.
