Dion-Jacobson (DJ) quasi-2D perovskite solar cells (PSCs) have attracted significant attention owing to its greater potentials in realizing efficient and stable quasi 2D PSCs as compared to Ruddlesden–Popper counterpart. To further enhance power conversion efficiency (PCE) and stability, the fabrication of methylammonium-free formamidinium (FA)-based DJ quasi-2D PSCs is highly desirable. Herein, we report a strategy for constructing gradient energy band alignment by achieving gradient Br doping (GBD) via in situ ion exchange reaction between I and Br in FA-based DJ quasi-2D PSCs. First, the gradient energy band alignment can facilitate carrier transport, extraction and transfer. Second, the improved crystallinity and reduced defect density due to recrystallization process are realized after FABr treatment. Finally, the incorporation of Br also contributes to increased device stability. The device with GBD achieves a much higher PCE of 16.75% than control device (13.78%), which is mainly as a consequence of a significantly boosted V from 0.970 V to 1.107 V due to suppressed bulk and interfacial nonradiative recombination. The unencapsulated device with GBD maintains 93% of its initial PCE after aging under the relative humidity range of 15–20% for 1600 h, and 91% after aging at 60 °C for 400 h.