To date, mixed-cation lead mixed-halide perovskite materials are the mainstream choice to serve as light-absorption layers when fabricating single-junction perovskite solar cells (PSCs) and PSC-based tandem solar cells with high efficiency and good stability. However, the universally adopted thermal annealing protocol (∼100 °C for 30, 60, or even 90 min) has issues when used for multi-cation mixed-halide perovskites, such as being time-consuming and involving the potentially severe loss of organic components from the perovskite surface. Herein, we develop a high-temperature inverted annealing strategy for post-treating CsFAMA triple-cation mixed-halide perovskite films. The inverted annealing method, with a heating time of only 5 min at 150 °C with the film side placed face-down, is demonstrated to exhibit superiority when compared with the generally adopted annealing approach with the film side placed face-up. Upon inverted annealing, the perovskite film shows significantly enlarged grains together with markedly reduced defect states, especially at its surface, arising from slowed solvent evaporation and the suppressed volatilization of organic halide components. Corresponding photovoltaic devices achieved a highest efficiency of 20.4% with a stabilized power conversion efficiency (PCE) of 19.8%. Furthermore, we demonstrate that this annealing strategy is also applicable to other perovskite materials, providing an alternative method for efficiently preparing high-quality perovskite films.