Luminescence, excitation spectra, and decay curves of afterglow of the phosphors of CaS:Bi3+ and CaS:Eu2+, Tm3+ with and without co-doping of Na+ were investigated. It war found that co-doping monovalent ion Na+ could efficiently reduce the afterglow and the persistent time in both samples. The results imply that the trapping centers responsible for afterglow are defect-related in both samples. Based on these results, it was expected that co-doping any trivalent ions, such as Y3+ and Al3+, into CaS:Eu2+ to substitute Co2+ could produce charge defects and create trapping centers Acquisition of strong afterglow and long persistent time in CaS:Eu2+,Y3+ and CaS:Eu2+,Al3+ is a strong support for the conclusion.