Biotin avidin system (BAS) is a new bioreaction amplification system developed in the late 1970s. With the advent of various biotin derivatives, BAS was soon widely used in various fields of medicine. A large number of studies have confirmed in recent years that the biotin avidin system can be combined with almost kind of markers for research success. The strong binding of biotin and avidin with high affinity and multi-stage amplification effect make BAS immunolabeling and related tracer analysis more sensitive. It has become a new technique widely used in the qualitative and quantitative detection of trace antigens and antibodies, as well as in localization observation research.