Quasi-two-dimensional (2D) organic-inorganic hybrid tin perovskites have emerged as promising alternatives to lead-based perovskites in thin-film photovoltaics because of their reduced toxicity and improved stability. However, the undesired small n-value 2D phases and disordered crystal orientation enormously restrict the efficiency of quasi-2D tin perovskite solar cells (PSCs) due to uncontrollable nucleation and crystal growth processes. Here, we propose a mixed pseudo-halide anion engineering approach by using acetate (Ac⁻) and tetrafluoroborate (BF₄⁻) anions to make quasi-2D Ruddlesden-Popper perovskites with a target formula of PEA₂FA₄Sn₅I₁₆. We find that the mixed Ac⁻ and BF₄⁻ anions can not only promote homogeneously distributed PEA⁺ cations in the precursor by effectively breaking the PEA⁺···PEA⁺ stacking but also retard the crystallization process by coordinating with unbonded SnI₂, thereby, significantly reducing small n-value 2D phases, improving the crystal orientation, and suppressing the Sn²⁺ oxidation. The resulting PSC exhibits up to 9% power conversion efficiency and 400 h stability.