Rare earth doped 12CaO·7Al2O3 (C12A7:RE with RE=Gd3+, Er3+, Ce3+, Tb3+, Eu3+, Eu2+) phosphors have been prepared and treated under different conditions to trap different anions in the nanocages of C12A7. The effects of the encaged anions on the optical spectroscopies of the RE doped C12A7 phosphors, including photoluminescence and color tuning, upconverted emissions, and cathodoluminescence, have been investigated. Electron paramagnetic resonance spectra are also studied in combination with the optical spectra. Crystal field modification of RE emission centers by the encaged anions is observed by both photoluminescence and electron paramagnetic resonance. A relationship between the ratio of green to red upconverted emissions of Er3+ and the amount of encaged OH− anions is established and the concentration of the OH− determined. Emission enhancement in C12A7:Ce3+,Tb3+ conductive phosphors is realized by a multiphase strategy, which has a potential application in field emission devices. Color tuning of C12A7:Eu3+/Eu2+ phosphor is also performed in the range from red to blue by controlling the sort and amount of encaged anions.