This study proposes a Professional Identity Cycle Model based on the Process Role Theory to explore the formation mechanism of clinical professional identity of undergraduate nursing students in three stages: role expectation，role apperception，and role practice. The present study also aims to explore the obstacles and facilitators in the process of professional identity formation and its outcome. The results indicate that occupational solidification，low occupational value and insufficient occupational support are the hindering factors for the formation of the professional identity of nursing students. Facilitators include self-expectation， nursing role models and clinical adaptability，eventually forming two kinds of professional identity results: participants’and bystanders’professional identity. The formation process of undergraduate nursing professional identity reflects the transformation process of nursing professionals toward modernization and the conflict between traditional social structure and modern individual consciousness among undergraduate nursing students. Although they have a professional identity dilemma in the role expectation and role understanding，most undergraduate nursing students still show a positive role practice attitude，which forms a spiral pattern of professional identity.