Mrs. Dalloway depicts a day in the life of the heroine Clarissa Dalloway in post WWI world. There is a subplot of Septimus accompanying the main plot of Clarissa Dalloway. Septimus, the second most important character is often compared with the heroine Clarissa Dalloway. There is always a controversial discussion about the relationship between Clarissa Dalloway and Septimus, although they never meet each other in the novel. By an in-depth exploration of the two characters’appearances, actions, inner thoughts from psychological perspective and by a study of the structure of the narrative concerning the plots of the two characters, it is reasonable to consider Septimus as the dark double of Clarissa Dalloway.