Disclosed are a slow-release inhibitor for high-magnesium sulfide mineral flotation and an application thereof, where the inhibitor is a nano colloidal particle of an alkaline earth fluoride such as CaF2 and BaF2 or a highly-reactive natural alkaline earth metal mineral powder. When applied to the flotation separation of a high-magnesium sulfide ore, the inhibitor can slowly release F ions to preferentially form a MgF2 layer on the magnesium-containing mineral surface, which provides a structure similar to MgF2 on a surface of oxidized gangue minerals such as magnesium oxide, changing surface electrical property of the magnesium-containing mineral, inhibiting heterogeneous coagulation of magnesium-containing minerals and sulfide ores due to electrostatic attraction and reducing entrainment, enveloping and agglomeration of gangue minerals to efficiently inhibit the flotation of oxidized gangue minerals such as magnesium oxide.