The ultraviolet-light-induced absorption (UV-LIA) coefficient change is investigated in congruent LiNbO3 crystals doped with Mg2+ of different concentrations. By measuring the UV-LIA coefficient change and the two-color recording sensitivity in congruent LiNbO3∶Mg, it is found that the UV-LIA coefficient change in congruent LiNbO3∶Mg has a threshold effect with respect to the Mg2+-doping concentration. It is only when the molar fraction of Mg2+ is more than 3.0% that the UV-LIA change from the near-UV to the near-infrared spectral regions appears. The threshold behaviour is further confirmed by the measurement of the two-color recording sensitivity. This threshold molar fraction is lower than the so-called optical damage-resistant threshold concentration of 4.6% in LiNbO3∶Mg. The UV-LIA absorption effect is assumed to be related to the decrease and even elimination of the antisite defect NbLi in LiNbO3∶Mg.