Abstract: Marine oligosaccharides, which are prepared by degradation of marine polysaccharides, are carbohydrate compounds composed of two to ten monosaccharide residues. Compared with marine polysaccharides, marine oligosaccharides have several advantages, such as higher water solubility, stronger biological activity, and easier absorption by the body. Marine oligosaccharides have a series of physiological activities, such as anti-tumor, antioxidation, lowering cholesterol, and immunomodulation. They can be widely used in food, medicine, and other fields. This paper summaries marine polysaccharides, preparation of marine oligosaccharides from marine polysaccharides by enzymatic methods, and the catalytic mechanism of the enzymes. Furthermore, three major marine polysaccharides (alginate, chitosan, and xanthan) and the catalytic mechanism of the three enzymes (β-elimination mechanism of alginate lyase, replacement mechanism of chitosanase, and degradation mechanism of xanthan enzyme) are introduced. It is expected to provide a theoretical reference for preparation and application of marine oligosaccharides, deficiencies and potential for future research of preparation of marine oligosaccharides from marine polysaccharides by enzymatic methods are discussed.