With its excellent comprehensive performance, Gd₃(Al,Ga)₅O₁₂:Ce (GAGG:Ce) scintillation crystal has broad application prospects. To accelerate the luminescence decay rate, Mg-codoped Gd₃(Al,Ga)₅O₁₂:Ce single crystals were grown by Czochralski method. The test results show that with the concentration of Mg²⁺ increasing, the scintillation decay of the crystal accelerates and the light output decreases. The conventional interpretation suggests that Mg²⁺ could convert part of Ce³⁺ into Ce⁴⁺ through charge compensation, and the Ce⁴⁺ luminesces more quickly. This study tries to discuss the mechanism of Mg-codoping in GAGG:Ce crystals from the perspective of antisite defect. Since the ionic radius of Ce is larger than Gd, the doping of Ce ions leads to a distortion of the crystal lattice near the luminescence center Ce_Gd. As a result of the distortion, the space of the adjacent octahedral sites become larger, and the antisite defects are more likely to form in these larger octahedral sites. Eventually each luminescence center Ce_Gd is surrounded by four antisite defects Gd_Al, which would capture carriers and delay the energy transfer from the matrix to the luminescence center. As the ionic radius of Mg is between Gd and Al, Mg_Al also prefers to form in those distorted octahedral sites, which inhibits the formation (or enrichment) of the antisite defect Gd_Al near the luminescence center Ce_Gd, and eventually reduces (or even eliminates) the adverse effects of the antisite defect on the luminescence center. XEL results show that with the increase of Mg concentration, the emission peak related to the antisite defect becomes weaker, which indicates that Mg could inhibit the formation of the antisite defect.