Cl-based salts are magical additives to control the perovskite crystallization and enhance film morphology. Especially for the I/Br halide wide-bandgap (WBG) perovskites, alloying Cl to form triple halide perovskites can effectively enhance their optoelectronic characteristics. However, the alloying mechanism of Cl into the I/Br-based perovskite lattice remains unclear. Here, we conduct a systematic in-situ photoluminescence (PL) exploration on the crystallization processes of I/Br-based WBG with Cl-based additives including MACl and PbCl₂. The results reveal that only the Cl from PbCl₂ is easy to incorporate into the I/Br-based perovskite lattice structure at the initial stage of perovskite nucleation. However, PbCl₂ incorporation results in the precipitation of excess PbI₂, which leads to unfavorable charge transport and decreased photostability. With co-incorporation of MACl and CsCl, the transition of crystal orientation during the annealing process is effectively regulated, significantly eliminating the accumulation of excess PbI₂. This improvement enhances phase homogeneity and reduces defect density. Consequently, the optimized WBG perovskite solar cell achieves a high efficiency of 21.58%, which is the highest value for 1.68 eV perovskite with bromine content lower than 10%. In addition, the operational stability is significantly enhanced, along with ameliorated burn-in aging behavior.