The Dion–Jacobson (DJ) perovskites solar cells (PSCs) demonstrate impressive humidity stability compared with 3D counterparts and have aroused extensive attention recently. However, it has not been investigated yet, how the humidity condition influences the photovoltaic performances. Herein, the effect of the humidity condition on the crystallization behavior, charge dynamics, photovoltaic and stability performances of BDAFAPbI DJ perovskite is systematically investigated. Humidity-assisted annealing process can lead to reduced trap concentration and fast charge transfer process between different-n DJ phases. The BDAFAPbI PSC with the assistance of moisture achieves an efficiency as high as 17.17%, which is the highest value ever reported for methylammonium-free DJ PSCs. In addition, the humidity-, thermal-, and photo-stability properties of the DJ film and devices are significantly boosted. Further, first-principles calculation indicates that water molecules occupy the iodide vacancies through a strong bonding with Pb ions during the crystallization process under humidity and increase the decomposition energy of the DJ phase, which corresponds to the improved photoelectronic and stability properties. These results show that humidity-assisted deposition of 2D/3D multidimensional perovskite can be further explored to reduce the high demand of the production environment and enhance the performances.