Despite increasing research interest in two-dimensional (2D) perovskites, the stability and spectral response range of the photodetectors based on 2D perovskites are yet far from satisfactory. In this work, a semiconductor heterojunction is constructed based on dimethyl itaconate (DI) treated (BA)₂PbI₄ microplates and CsPb(Br_x/I_1−x)₃ nanocrystals. The DI treatment not only passivates the defects but also blocks the moisture, resulting in improved stability and suppressed defect trapping. Meanwhile, the type-II heterojunctions facilitate the separation of electron–hole pairs. As a result, the photodetector based on the DI-(BA)₂PbI₄/CsPb(Br_x/I_1−x)₃ exhibits a responsivity up to 209 nA/mW and a detectivity up to 5.9 × 10⁸ Jones with a broad spectral response range of 400–600 nm. Furthermore, after storage in the ambient environment for 10 days, the responsivity retains about 70%, which is significantly ameliorated compared to the photodetector based on bare (BA)₂PbI₄ (drops more than 90%). Therefore, this work demonstrates that surface passivation and 2D/three-dimensional heterojunctions are promising strategies to improve responsivity, broaden spectral response range, and enhance stability of photodetectors based on 2D perovskites.