Limited by high chloride ion migration, lots of low-grade FGD gypsum cannot be effectively used in gypsum products. The influence and mechanism of Q phase (Ca20Al26Mg3Si3O68) on the migration of chloride ion in FGD gypsum were studied. The results show that the hydrates of gypsum-Q phase in which 10%~30% (mass fraction) of Q phase is blended are mainly calcium sulfate dihydrate, ettringite and gibbsite. The chloride ion immobilization capacity and absolute dry compressive strength of the FGD gypsum sample are greatly improved. Adding 30% Q phase achieves the best performance improvement compared with FGD gypsum. At this time, the chloride ion immobilization capacity reaches 19.55% and the absolute dry compressive strength of the gypsum increases 51.5%. Combining XRD, SEM and thermal analysis, the mechanism of ettringite and gibbsite in the FGD gypsum-Q phase hydrates on the immobilization of chloride ion and the improvement of the mechanical properties of the gypsum are explored.