"The Kite Runner" is a representative novel by Afghan-American writer Khaled Hosseini. In this novel, Hassan's loyalty moved countless readers while Amir's betrayal shocked and even angered many readers. In the researcher's view, all behaviors of the protagonist, Amir, such as his betrayal, guilt and redemption, are closely related to his psychological changes. From mistrust to doubt, guilt to inferiority, self-accusation to role confusion, and from being willing to love to devotion, different psychological states lead to different emotional needs and behaviors. Admittedly, the whole psychological process is actually a journey of growth for Amir. By employing Erikson's psycho-social development theory to analyze Amir's psychological change in different stages, this paper aims to reveal Amir's inner world so that we can have a better understanding of the reasons for Amir's sin and cowardice.