Red emitting Ca2GeO4 ∶Eu3+ phosphors are synthesized by solid state method. The ultraviolet and vacuum ultraviolet excited photoluminescence properties are investigated in detail. The phenomenon of reduction of Eu3+ ions in air and the color switches are discovered and explained. The Ca2GeO4 ∶Eu3+ presents intense excitation intensities at 163—230, 301 and 466 nm, showing the potential applications in plasma display panels and light emitting diodes. The excitation spectra are studied to identify the photoluminescence mechanism of Ca2GeO4 ∶Eu3+ . First principles calculation within the local density approximation of the density functional theory is used to calculate the electronic structure and the linear optical properties of Ca2GeO4.