Transition to school is a critical period in child development and has raised great concern among contemporary studies. However, most studies focus on vertical transition and only a few investigate vertical and horizontal transitions as a whole. To bridge the gap, this paper explores how an immigrant Chinese child makes the horizontal transition between dominant institutions within a bicultural-context and the vertical transition from preschool to the primary school period as a whole unity, and how the child learns and develops throughout this trajectory. The data (46 hours) being analysed are selected from observations and interviews. Drawing upon the cultural-historical theory, this paper argues that: (1) transition process is not linear, (2) every child has his/her unique transition trajectory and mapping the trajectory is a way to identify the child's challenges and needs, and (3) culture is not a separate factor, but rather is embedded in the social situation of development in which the child is situated.