The invention belongs to the technical field of environmental-heavy-metal pollution treatment and discloses a potassium-type farmland passivation agent based on raw materials of molybdenum ore tailings and a preparation method and application of the passivation agent. The method includes: subjecting the molybdenum ore tailings to acid pickling prior to adding an alkaline potassium compound; performing common heating in a muffle and then dissolution by adding water; taking a supernate prior to regulating material ratio by adding sodium metaaluminate and water glass; performing reaction in a reactor to obtain the farmland passivation agent after passivation of reaction products after the reaction is completed. The farmland passivation agent is quite good in passivation effect, and CaCl2 extraction-state heavy metal content can be reduced by more than 95%; the prepared farmland passivation agent is produced under the alkaline environment, the pH value of acid soil is increased by at least 2 units during passivation of heavy metal, soil acidification is improved, and the farmland passivation agent can be applied to the field of farmland repairing; waste of the raw materials of the molybdenum ore tailings is fully utilized, quality of the product is improved, and meanwhile, the purpose of dealing with waste by waste is achieved.