Abstract: Alginate oligosaccharide (AOS) is a kind of low molecular polymer, which is composed of 2~25 monosaccharides (mannuronic acidandguluronic acid) by glycoside bond. Compared with alginate, AOS, which has the characteristics of low viscosity, good water solubility, easy absorption and strong stability, meanwhile which has many biological activities, such as food preservation, anticoagulation, anti-tumor, blood pressure and lipids reduction, promoting plant growth, inducing plant resistance, has become a research hotspot in the field of marine polysaccharides. The purity, structure and properties of alginate oligosaccharides are closely related to their preparation, separation and characterization. This paper reviews the preparation methods of alginate oligosaccharides, such as physical degradation, chemical degradation, enzymatic degradation and fermentation; The separation technology of alginate oligosaccharides, such as sedimentation separation, membrane separation and chromatography; The structure characterization methods of alginate oligosaccharides, such as Thin Layer Chromatography (TLC), Infrared Absorption Spectroscopy(FTIR), Circular Dichroism (CD), Mass Spectroscopy (MS), Nuclear Magnetic Resonance Spectroscopy (NMR), in order to provide reference or further research and application of AOS.