Low‐bandgap mixed tin–lead perovskite solar cells (PSCs) have been attracting increasing interest due to their appropriate bandgaps and promising application to build efficient all‐perovskite tandem cells, an effective way to break the Shockley–Queisser limit of single‐junction cells. Tin fluoride (SnF2) has been widely used as a basis along with various strategies to improve the optoelectronic properties of low‐bandgap SnPb perovskites and efficient cells. However, fully understanding the roles of SnF2 in both films and devices is still lacking and fundamentally desired. Here, the functions of SnF2 in both low‐bandgap (FASnI3)0.6(MAPbI3)0.4 perovskite films and efficient devices are unveiled. SnF2 regulates the growth mode of low‐bandgap SnPb perovskite films, leading to highly oriented topological growth and improved crystallinity. Meanwhile, SnF2 prevents the oxidation of Sn2+ to Sn4+ and reduces Sn vacancies, leading to reduced background hole density and defects, and improved carrier lifetime, thus largely decreasing nonradiative recombination. Additionally, the F− ion preferentially accumulates at hole transport layer/perovskite interface with high SnF2 content, leading to more defects. This work provides in‐depth insights into the roles of SnF2 additives in low‐bandgap SnPb films and devices, assisting in further investigations into multiple additives and approaches to obtain efficient low‐bandgap PSCs. In‐depth insights into the roles of tin fluoride (SnF2) additive in low‐bandgap mixed tin (Sn)‐lead (Pb) perovskite and efficient solar cells are provided. The growth mode of the film, highly oriented topological growth, and reduced Sn2+ oxidation are achieved via proper SnF2 doping. Additionally, the accumulation of F− at hole transport layer/perovskite interface is shown at higher SnF2 content, leading to more defects.