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-1)PbnI3n+1　(where　PDA　is　1,3-propylenediamine,　MA　is　methylamine,　nominal　n　=　4).　The　best-performance　device　exhibits　a　high　detectivity　of　4.57　X　10(11)　Jones,　a　responsivity　of　0.25　A.W-1　at　480　nm,　a　low　dark　current　density　of　9.60　X　10(-4)　mA.cm(-2),　and　a　remarkable　on/off　ratio　of　3.10　X　10(5).　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.