Ca3Sc2Si3O12:Ce3+ (CSS:Ce) green phosphors used for white light-emitting diodes (LEDs) are synthesized and codoped with Al3+ via a solid-state reaction method. The crystal structure and vibrational modes are analyzed by X-ray diffraction, Fourier transform infrared spectroscopy, and Raman scattering spectroscopy. The energy transfer behavior and optical performance are characterized by photoluminescence and excitation spectra, quantum efficiency, and time-resolved photoluminescence. The incorporation of Al3+ into CSS:Ce can inhibit the formation of the impurity phases Sc2O3 and CeO2, improve crystallinity, and enhance the photoluminescence intensity as well as quantum efficiency. The substitution of Sc3+ with Al3+ increased the crystal field splitting of Ce3+ and resulted in the red shift of photoluminescence. The results show that Ca3Sc2−xAlxSi3O12:Ce3+ has high quantum efficiency, making it a promising green phosphor that can be collocated with a commercial 450 nm blue LED and a red phosphor for solid-state lighting applications.