A two-dimensional (2D) organic-inorganic hybrid perovskite (OIHP) material is considered as a promising candidate for a long-term stable photodetector owing to its outstanding phase and environmental stability. Herein, we demonstrate a perovskite photodiode based on the DJ phase 2D perovskite (PDA)(MA)_n-1Pb_nI_3n+1 (where PDA is 1,3-propylenediamine, MA is methylamine, nominal n = 4). The best-performance device exhibits a high detectivity of 4.57 × 10¹¹ Jones, a responsivity of 0.25 A×W⁻¹ at 480 nm, a low dark current density of 9.60 × 10⁻⁴ mA×cm⁻², and a remarkable on/off ratio of 3.10 × 10⁵. The unencapsulated device can maintain 95% of the initial photocurrent density after 90 days under an ambient atmosphere with relative humidity (RH) of 65%, demonstrating its improved stability than the 3D counterpart. The excellent stability of the photodiode based on 2D perovskite promises its bright commercial application future.