The invention discloses amphiphilic block inner quaternary ammonium salt as well as a preparation method and an application thereof. The structural general formula of the inner salt is represented by the formula I. By an atom transferring free radical polymerization method, a polymer containing a tertiary amine radical is led into a molecule containing an acid radical, so that the same molecule contains the acid radical and the alkaline radical at the same time, and the acid radical and the alkaline radical can be combined to form the inner quaternary ammonium salt. The inner salt has the characteristics of high efficiency, low toxicity, high solubility and high biodegradability and also has the characteristics of temperature resistance, shearing resistance, hypersalinity resistance and high water solubility; at the high temperature and the hypersalinity, the inner salt still has an extremely good solution viscosity reducing effect; and the amphiphilic block inner quaternary ammonium salt can well emulsify and solubilize crude oil and can be widely applied to the field of petrochemical engineering, particularly used as an oil expelling agent for tertiary oil recovery in an oil field.