Purpose: This paper aims to contribute experience and ideas of the transformational leadership, not only for the principal want to improve leadership himself (herself), but also for the school at critical period of improvement, through summarizing forming process and the problem during the course and key factors that affect the course. Design/methodology/approach: Using case study method, we draw some conclusions of improvement of principal leadership skills, by choosing two typical cases of the transformational leadership of principals in primary schools, rethinking the practice of school improvement profoundly. Findings: There have been three periods: embryonic stage, basically forming stage, and mature stage. That discovering problems, understanding the relationship between the problems and finding solutions, are the major consideration in the process. The transformational leadership skills of principal can be seen in forming the ideas, building shared vision, power sharing, gaining credence and experiencing success. Practical implications: Providing references for the principal to recognize the importance of transformational leadership during the school improvement, in order to force the principal to improve their transformational leadership in practice. Theoretical significance: At first, put forward that transformational leadership is the core of leadership; prove that principal's transformational leadership is very important in the school development through the fact that home-school cooperation study; emphasize that principal's transformational leadership should play a key role during the critical period of school improvement. Principal transformational leadership's work should base on different school members' actual situation and different school periods' fact, and then find different solutions.