The present disclosure discloses methods for preparing an in-situ nano emulsifier. The preparation method may include obtaining a solution by dissolving hydrated ferric chloride (or ferric chloride) and hydrated ferrous sulfate (or ferrous sulfate) into deionized water at a first temperature; obtaining an ethanol solution dissolved with an oil-soluble surfactant by dissolving an oil-soluble surfactant into ethanol at a second temperature; and adding a certain volume of ammonia water and the ethanol solution dissolved with the oil-soluble surfactant in the solution to obtain an in-situ nano emulsifier. The in-situ nano emulsifier may be applied to development of the oil reservoir through an application process. The application process may include preparing a nano emulsifier solution and a surfactant solution; sequentially injecting the surfactant solution and the nano emulsifier solution into a formation; and injecting a spacer liquid slug into the formation for replacement.