Immunoassay is an indispensable analytical method in modern medicine. At present, most immune technologies have the problems such as high detection limit, low sensitivity, and poor reliability in complex systems, which are difficult to meet the requirements of precision medicine. Single molecule immunoassay, also known as digital immunoassay, can quantify by measuring the number of immune complexes. It is an ultrahigh sensitive immunoassay technology, which can meet the needs of high sensitive detection such as early detection of major diseases and detection of rare antigens in blood. This paper introduces the basic theory, classification methods and specific applications of single molecule immunoassay, focuses on the research status in the field of single molecule counting immunoassay at home and abroad, and discusses the prospects of the applications of this immunoassay technology in medical clinical diagnosis.