The yellow-emitting phosphor [Ca3−(x+0.06)LuxCe0.06](Sc2−yMgy)Si3O12 obtained from Lu3+ and Mg2+ co-modified green-emitting silicate garnet Ca3Sc2Si3O12:Ce3+ (CSS:Ce3+) exhibits promising applications for white LEDs. In this paper, we discuss the effect of charge balance on the garnet structure formation. The changes of bond length and covalence caused by the replacement of Lu3+ and Mg2+ for Ca2+ and Sc3+ are analyzed. The shift of the Ce3+ emission and excitation can be attributed to the combined results from crystal field splitting effect and centroid shift of Ce3+ 5d levels. Thermal stability is analyzed according to configurational coordinate diagram.