AbstractControlling the crystallization to achieve high‐quality homogeneous perovskite film is the key strategy in developing perovskite electronic devices. Here, an in situ dynamic optical probing technique is demonstrated that can monitor the fast crystallization of perovskites and effectively minimize the influence of laser excitation during the measurement. This study finds that the typical static probing technique would damage and induce phase segregation in the perovskite films during the excitation. These issues can be effectively resolved with the dynamic probing approach. It also found that the crystallization between MAPbI₃ and MAPbI₂Br is strikingly different. In particular, MAPbI₂Br suffers from inefficient nucleation during the spin‐coating that strongly affects the uniform crystal growth in the annealing process. The commonly used pre‐heating process is found at a lower temperature not only can further promote the nucleation but also to complete the crystallization of MAPbI₂Br. The role of further annealing at a higher temperature is to facilitate ion‐dissociation on the crystal surface to form a passivation layer to stabilize the MAPbI₂Br lattices. The device performance is strongly correlated with the film formation mechanism derived from the in situ results. This work demonstrates that the in situ technique can provide deep insight into the crystallization mechanism, and help to understand the growth mechanism of perovskites with different compositions and dimensionalities.