Tin (Sn)-based perovskite solar cells (PSCs) have attracted much attention because they are more environmentally friendly than lead-based PSCs. However, the fast crystallization of Sn-based perovskite films and the easy oxidation of Sn²⁺ to Sn⁴⁺ hinder the improvement of their efficiency and stability. In this work, ethylammonium bromide (EABr) was added to methylammonium tin iodide (MASnI₃) perovskite precursor solution to regulate the crystallization dynamics and improve the film morphology. The results show that the large EA⁺ ions slow down the crystallization process of Sn-based perovskites and form a smooth perovskite film with high crystallinity, while the added Br⁻ anions further improved the crystallinity and orientation of the perovskite film. Under the combined action of EA⁺ and Br⁻ ions, the as-produced PSCs achieved a champion power conversion efficiency (PCE) of 9.59%. The EABr additive also retarded the oxidation of Sn²⁺, and the solar cell device maintained 93% of its initial efficiency after 30 days in a nitrogen-filled glove box without being encapsulated. This work provides a new strategy for the realization of high-efficiency Sn-based PSCs.