Near-infrared (NIR) light has been used for in vivo deep imaging in the biomedical field owing to its high permeability in biological tissues. Conventional fluorescence bioimaging requires simultaneous optical excitation, which can induce undesired results such as autofluorescence. In the present study, we report a polymer-based NIR afterglow fluorescent complex by using NIR dye, LDS 821, and SrAl2O4: Eu, Dy powder. To obtain the complex, SrAl2O4: Eu, Dy was first mixed with LDS 821 solution and dried at 65 °C for 12 h. Then, they were further mixed with pentaerythiritol tetraacrylate (PETA) and photopolymerization initiator and then subjected to phopolymerization reaction. Poly-PETA was effective for not only retain both the dye and inorganic powder but also preventing the denaturation of SrAl2O4: Eu, Dy by water. The obtained polymer-based complex emits NIR afterglow fluorescence (wavelength: 760−780 nm) with a long fluorescence lifetime (5.0 min). This material design will provide a choice of obtaining NIR luminescent imaging phosphors for deep tissues without simultaneous light irradiation during image acquisition.