Tin (Sn) is a potential replacement for lead in organic–inorganic perovskite solar cell (PVSC) technology due to the attractive optoelectronic properties of Sn-perovskites. However, the poor stability of Sn²⁺ ions is still the main hindrance for achieving highly stable and efficient Sn-based PVSCs. In this work, anilinium hypophosphite (AHP) is introduced as a co-additive into a Sn-absorber perovskite precursor containing the tin fluoride (SnF₂) additive. The incorporation of the AHP additive not only prohibits the phase separation of SnF₂ but also passivates the perovskite films and finally results in absorbers with superior structural and optoelectronic properties. The underlying reason is related to the formation of a double-salt complex (Sn(H₂PO₂)₂·SnF₂) through the interaction between AHP and SnF₂. The high quality of the absorber film induced by AHP delivers an improved device power conversion efficiency (PCE) with a higher open-circuit voltage than those without the AHP additive. More importantly, the device with the AHP additive can retain up to 97% of its PCE after 30 days of storage in a glovebox.