The invention discloses a preparation method of an agomelatine intermediate. The preparation method comprises steps as follows: firstly, 1-bromo-7-methoxynaphthalene is dissolved in an organic solvent, and a mixed solution is formed; elementary alkali metal and an initiator are added to a reaction bottle, the organic solvent is added to the reaction bottle and stirred uniformly, the prepared mixed solution is dropwise added slowly, when a reaction liquid is boiled slightly, the organic solvent is supplemented into the reaction bottle and stirring is started, the prepared mixed solution is dropwise added continuously, and a reaction liquid of a 1-bromo-7-methoxynaphthalene Grignard reagent is obtained after the heat preservation reaction; bromoacetonitrile is added to the organic solvent, a bromoacetonitrile mixed liquid is prepared, the prepared bromoacetonitrile mixed liquid is dropwise added slowly to the reaction liquid of the Grignard reagent, and (7-methoxy-1-naphthyl) acetonitrile is obtained after the heat preservation reaction and aftertreatment. With the adoption of the method, industrial production can be realized, 1-bromo-7-methoxynaphthalene is taken as a starting material, the cost can be saved, the product quality is stable, the yield is high, and the preparation method is applicable to large-scale industrial stable production.