Various popular large organic cations have been extensively used as the essential additives in the perovskite precursor solution due to their satisfactory passivation effect but may produce the low-n value (n ≤ 2) 2D perovskite phases with undesired distribution. Meanwhile, the remaining easy oxidation of Sn²⁺ and the p-type self-doping in the perovskites are also detrimental to the ultimate photovoltaic properties and stability of tin (Sn)-based perovskite solar cells (PSCs). Here, 3AMPYSnI₄ crystals (3AMPY = 3-(aminomethyl)pyridinium)) are designed and applied to adjust the crystallization process and the phase distribution of the Sn-based perovskite. Consequently, the strong coordination interaction between 3AMPY²⁺ and 3D perovskite components and the introduced nucleation sites by 3AMPYSnI₄ crystals not only decreases the low-n value 2D phase and increases 3D perovskite phase, but also inhibits the oxidation of Sn²⁺ and the self-p-doping in the Sn-based perovskites, resulting in lower trap density and non-radiative recombination loss, faster carrier extraction and transfer, and higher stability for 2D-3D Sn-based PSCs. As a result, the optimized devices deliver an increased power conversion efficiency from an initial 10.91% to 13.28% and retain 96.0% of their original performance for more than 3000 h in the nitrogen (N₂) atmosphere.