A new red-emitting phosphor, Eu3+ doped Al5BO9, was prepared for the first time by calcining the precursor of K2[Al(B5O10)]·4H2O:Eu3+ which was synthesized by a facile hydrothermal route. The obtained samples were characterized by energy dispersive x-ray spectrometer, x-ray powder diffraction, IR, scanning electron microscopy, photoluminescence, and photoluminescence excitation spectrum (PLE). Moreover, the influences of Eu3+ doping concentration, calcination temperature, and calcination time on the luminescence properties of Al5BO9:Eu3+ phosphor were also investigated. The phosphor with optimal luminescent intensity and the higher red/orange ratio was obtained by sintering the precursor at 1300 °C for 5.5 h, with 5% doping concentration, in which its luminescent decay lifetime and quantum efficiency were also measured. It is also found that the phosphor prepared by conventional solid-state reaction method exhibits the dominant transition at 591 nm (orange) with the lower color purity, while the phosphor prepared by the present precursor method exhibits the dominant transition at 615 nm (red) with the higher color purity, which indicates that this is a good method for preparing Al5BO9:Eu3+ red phosphor.