Near infrared-II (NIR-II, 1 000—1 700 nm) imaging with high penetration tissue depth and signal-noise ratio has attracted wide interest in biomedicine. As a two-dimensional (2D) material with narrow band gap, the band structure of layered black phosphorus, as an important characteristic of electronic structure, determines the electronic transport and infrared optical properties, which show great potential in NIIR-II imaging. Here, the electronic structure and NIR-II optical properties of black phosphorus have been investigated in detail by employing the generalized gradient approximation + U (GGA+U) correction based on density functional theory (DFT). First, we performed the band structure and density of states for different layers of black phosphorus. From the electronic structures, the location of valence band maximum didn’t shift obviously, and the position of conduction band minimum shifted downward gradually, inducing the band gaps decreased gradually with the increasing layer number. While the layer number increased to 5, the behaviour of electronic structure was very similar to that of the bulk black phosphorus. Then, we calculated the NIR-II optical properties, and found the optical band gap of black phosphorus also showed layer dependent properties. From a single layer to 5 layers, the optical band gap changed from 1.71 eV to 0.92 eV. It is noting that black phosphorus also showed the significant optical absorption in NIR-IIa (1 300—1 400 nm) and NIR-IIb (1 500—1 700 nm) windows. Especially, the NIR-II optical absorption can be enhanced with increasing the layer number to 5, indicating promising photoresponse materials in NIR-II imaging.