[en] Anion exchange reactions in lead halide perovskites provide post-treatment methods to functionalize and modulate their photoelectric properties. However, the mechanism of anion exchange reactions is still fiercely debated. Herein, by investigating time-dependent in situ PL spectra after mixing CsPbBr₃ and CsPb(Br_0.09/I_0.91)₃ perovskite nanocrystals (PNCs), the anion exchange process is divided into three stages, i.e. photon reabsorption process, initial anion exchange, and adequate anion exchange. This model is broadly applicable to different situations, ignoring the mixing order or molar ratios between Br and I. This work provides a dynamic model to describe the anion exchange of CsPbX₃ PNCs, which will not only add fundamental knowledge to the stability of PNCs, but also guide the post-treatment of PNCs