The Source of infectious bacteria is the first problem in provension of infection considered by the medical workers. In this paper, with the method of bacteria plasmid analysis, chromosome DNA restriction fragment length polymorphism analysis, drug resistance experiment (antibacterial drug, sterilizing drug) and so on. We confirmed that staphylococci of nosocomi is mainly from air; but the gram-negative bacteria is mainly from the patient's pharyngeal portion; and cause hospital infection indirectly. The drug resistance of gram-negative from air was lower than the same gram-negative bacilli's isolated from the patient. Moreover, kill bacteria the infectious strain is same strain in air. Above-mentioed results provided the important basis for control of the hoslpital infection.