<p>All-perovskite tandem solar cells (TSCs) have garnered significant attention due to their high efficiency potential. Among these, Sn-Pb perovskite solar cells (PSCs) play a crucial role in all-perovskite tandem configurations. However, the Sn²⁺ in Sn-Pb perovskites is prone to oxidation, leading to severe p-type self-doping and significant non-radiative recombination. Additionally, the uneven crystallization of Sn-Pb perovskites can result in non-uniform crystallization of the perovskite films, generating a substantial number of defects. In this study, we introduce sulfaguanidine (SG) into the perovskite precursor solution. The strong binding energy between SG and tin(II) iodide results in a delayed release of tin iodide during the crystallization process. Furthermore, the incorporation of SG significantly reduces the charge transfer between O₂ and Sn²⁺, thereby increasing the energy barrier for Sn²⁺ oxidation and effectively suppressing its oxidation. Consequently, the single-junction Sn-Pb PSCs exhibit a stable power conversion efficiency (PCE) of 22.70%. We further integrate the Sn-Pb perovskite into a two-terminal allperovskite TSC, achieving a PCE of 28.73%. Furthermore, the operational stability is further assessed by tracking the maximum power point (MPP) under AM 1.5G conditions. The encapsulated SG-modified tandem devices maintain 85.40% of their initial PCE after 250 h, demonstrating a significant improvement in stability.</p>