Hepatitis B surface antigen (HBsAg) is one of the five indicators of hepatitis B. A positive HBsAg indicates that the body is infected with hepatitis B virus. The intact hepatitis B virus is spherical in shape and consists of an envelope and a nucleocapsid, which contains HBsAg, glycoproteins and other components. There is a protein on the surface of the virus that can act as an antigen and stimulate the body’s immune system to produce hepatitis B surface antibodies, so it is a marker of the presence of the hepatitis B virus and is the first of the five hepatitis B items (i.e., hepatitis B two-to-one). The earliest HBsAg positivity in the blood of adults infected with the hepatitis B virus is 1-2 weeks at the earliest and 11-12 weeks at the latest. Acute self-limiting hepatitis B virus infection with HBsAg in the blood lasts more than 1-6 weeks and up to 20 weeks, and can persist for years or even for life in asymptomatic carriers and chronic patients. HBsAg itself is only antigenic, not infectious, if the test shows a positive “surface antigen”, it means that the body is infected with hepatitis B virus, but HBsAg does not reflect the status of hepatitis B virus replication in the body, so it is not an indicator of hepatitis B infectiousness. If HBVDNA is positive, it means that there is hepatitis B virus replication and infectiousness; if there is both abnormal liver function and positive HBVDNA, it indicates that there is hepatitis virus activity and antiviral treatment should be carried out. In summary, hepatitis B surface antigen is a marker for the presence of hepatitis B virus, and its positivity represents being infected by hepatitis B virus, and patients should be promptly diagnosed and treated for the best treatment time.