A novel and simple pathway for revealing the reaction mechanism of perovskite nanocrystals (NCs) is reported, which involves injecting a halogen source into a solution of Pb and Cs sources so that the nucleation and growth kinetics can be decoupled. Unlike previously reported methods, there is no pre-existing [PbX6]4- octahedron in the reactant because of the separation of the halogen and lead, and it is formed only afterwards when the halogen source is injected. Since the formation speed and the number of [PbX6]4- octahedral domains are crucial to the nucleation process, the formation mechanism can be revealed by controlling the reaction process. The halogen source is identified to be the key factor in controlling the nucleation process of the perovskite NCs. Besides, the perovskite NCs with different dimensionalities can be synthesized by tuning the feed ratio.