The Scarlet Letter is a psychological romance,which is full of delicate inward description.Based on the author Hawthorne’s excellent effectiveness in the psychological description,this thesis uses Sigmund Freud’s theory of personality structure and Anna Freud’s theory of psychological defense mechanisms to analyze Dimmesdale,a leading character in the novel,and to show the development of Dimmesdale’s psychology and the reasons of his final death. The Scarlet Letter is a psychological romance, which is full, in the context of the psychological description, this author Hawthorne’s excellent effectiveness in the psychological description, this thesis uses Sigmund Freud’s theory of personality structure and Anna Freud’s theory of psychological defense mechanisms to analyze Dimmesdale, a leading character in the novel, and to show the development of Dimmesdale’s psychology and the reasons of his final death.