Structural phase transitions in semiconductor nanocrystals strongly influence their properties, yet atomic-scale pathways remain difficult to clarify in low-dimensional systems. In this presentation, we study off-stoichiometry-induced phase transitions in two-dimensional (2D) CdSe quantum nanosheets using in-situ transmission electron microscopy. Atomically flat CdSe nanosheets initially exhibit a hexagonal wurtzite structure and subsequently transform into a cubic zinc blende phase. Selenium loss at the nanosheet surfaces induces atomic reconstruction of the wurtzite (11̅20) planes, which show structural correspondence with the zinc blende (001) planes, driving the transformation. During this process, dynamic atomic rearrangements result in domain merging and separation. This work provides direct insight into phase transition pathways in 2D semiconductor nanocrystals. These findings highlight the importance of surface-driven atomic reconstruction in governing phase stability at the nanoscale.