Pathogenicity island is a gene cluster that have special structure and located in bacterial chromosome,which function is code productions that associated with bacterial virulence and metabolism. Type Ⅳ secretion system（T4SS）is an important export pathway used by Gram-negative pathogenic bacteria to inject bacterial proteins into the cytosol of eukaryotic host cells.Cag and type Ⅳ secretion system coded by cag, a pathogenicity island(PAI) of Helicobacter pylori, were all the key factors to cause diseases. While which was a burning question in related fields recently, and would also be a new target for medical treatment. This review summarized the recent studies on it.