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The environmental stability of black phosphorus (BP) is very important for its practical application. Here, we studied the protective effect of 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA) passivation layer on the BP against air exposure with photoemission spectroscopy (PES) and density functional theory (DFT) calculations. PES characterization manifests that NTCDA effectively extracts electrons in the BP valence band (VB) and increases the work function (WF) of BP. The PES results reveal a straddling gap (type-I) band alignment at the interface, accompanying with an interface dipole of 0.13 eV pointing from NTCDA to BP and band bending of 0.17 eV in BP. The charge transfer of 0.32 electron per molecule from BP to NTCDA is further confirmed by DFT calculations. Air exposure experimental results demonstrate that NTCDA can improve the environmental stability of BP and slow down the surface oxidation and degradation. Our work deepens the understanding of the passivation mechanism of BP and provides more experimental and theoretical insights for BP’s surface modification and future applications in Two-dimensional (2D) devices.