Abstract: Chitooligosaccharides are the only basic oligosaccharides in nature with -NH₂ and -OH functional groups. They are easily soluble in water and have extremely high biological activities such as antibacterial, anti-inflammatory, anti-oxidant, and anti-cancer. Chitooligosaccharides can be combined with metal ions, proteins, polysaccharides, lipids, and flavonoids through coordination complexation, electrostatic interaction, transamination, Maillard reaction, amidation, esterification, and carboxylation of chitooligosaccharides. Chitooligosaccharide complex has good stability and high water solubility, and can have the biological activity characteristics of chitosan oligosaccharide and complex factor at the same time, which effectively broadens the application range of the complex and enhances the effect of function. In addition, studying on the new compound method and the safety of chitooligosaccharide complex, which will become an important research direction. This article summarizes the preparation methods and compound objects of chitooligosaccharide complexes, the efficacy and application research of chitooligosaccharide complexes in functional foods, agriculture, biomedicine and other fields in recent years in order to provide a reference for the further study of chitooligosaccharide complexes.