Preparation method of a compound of general formula (I) comprises the following steps: (1) a compound of general formula (II) reacts with a formylating agent in an aprotic solvent at a temperature between −20° C. and 200° C. in the presence of a Lewis acid, then an organic base is added to promote the reaction to obtain an intermediate product; (2) the above intermediate product reacts with a methylating agent in the presence of an alkali at a temperature between −20° C. and 100° C. to obtain the compound of formula (I).