Trivalent terbium-doped strontium aluminate (SrAl2O4:Tb3+) nanoparticles were synthesized via the sol-gel combustion technique, and the green photoluminescence (PL) and afterglow were evaluated to clarify the afterglow mechanism of SrAl2O4:Tb3+. The green PL of SrAl2O4:Tb3+ with characteristic emissions at 488, 543, 586, and 622 nm indicated that Tb dopant acts as the luminescent center of the PL. Contrarily, the green afterglow of SrAl2O4:Tb3+ was a broadband spectrum with its peak centered at around 520 nm, but no traces of Eu were found in SrAl2O4:Tb3+ phosphors within the detection limit of 1 mu g/g. The band structures and density of states of SrAl2O4:Tb3+ were calculated within the framework of density functional theory. Both the ground state of Tb3+ dopant and the trap levels of oxygen vacancy were quantitatively determined in the band gap of SrAl2O4. Our results suggest that the deep electron trap of oxygen vacancy in the host acts as the luminescent center of the green afterglow from SrAl2O4:Tb3+. A possible afterglow mechanism is proposed to shed fresh light on the green afterglow of SrAl2O4:Tb3+.