The light-yield enhancement and decay-time shortening by Ca codoped with Lu₂SiO₅:Ce have been observed in experiment. However, the underlying mechanisms remain enigmatic. First-principles calculations are performed to gain insight into the effect of Ca codoping. We show that first, the formation of CaLu−VO complex defects is favored, which causes the dissociation of CeLu−VO; therefore, the nonradiative decay of Ce via VO is suppressed, leading to light-yield enhancement. Secondly, the downward shift of the Fermi level by the codoping effect causes the observation of Ce⁴⁺ ions, and the electron trap depth of VO decreases by the combination with Ca; thus, the trapping/detrapping rates increase, leading to decay-time shortening. Hence, our study has demonstrated an effective way to gain insight into the mechanisms for tuning properties of optical materials by codoping. The principles of the obtained mechanisms provide guidance for designing and optimizing a broad range of functional materials. Copyright © 2023 American Physical Society.