Tb2.94MgxAl5−2xSixO12:Ce3+0.06 (TMASG: Ce) were synthesized by high temperature Solid-State Reaction (SSR) at different temperatures from 1650°C to 1550°C. Samples with x = 0.1, 0.2, 0.3, 0.6 were fabricated and characterized using powder X-ray diffraction (XRD) and room-temperature photoluminescence spectroscopy. In this work, the excitation and emission spectra indicate that the phosphor can be excited effectively by the blue light (450–550 nm) and exhibit a satisfactory orange–yellow emission from 550 to 575 nm. The reasons of the red-shift of the phosphor have been discussed. A crystal field analysis of the Ce3+ 5d energy levels is conducted to explain the correlation between the TMASG:Ce lattice expansion and the Ce3+ energy level changes. The chromaticity coordinates of the phosphors are tuned from (0.465, 0.527) to (0.512, 0.483). This indicates a prospective for application of this novel phosphor in white light emitting diodes (LEDs).