The invention discloses an alkylated solid acid catalyst and a preparation method thereof, and relates to an alkylated solid acid catalyst for a process of preparing mesitylene by mixed C9 aromatic hydrocarbon through alkalation separation and a preparation method thereof. The catalyst has higher alkylation conversion rate of the mixed C9 aromatic hydrocarbon and yield of the mesitylene under high temperature and normal pressure conditions. The catalyst takes heteropoly acid as a main active constituent, modified zirconium oxide as a carrier and alkali metal or rare earth element metal as a modifying agent; and calculated as the weight of zirconium oxide, the adding amount of the alkali metal is between 0.01 and 10 weight percent, the adding amount of the rare earth element metal is between 0.01 and 10 weight percent, the loading capacity of the heteropoly acid is between 5 and 30 weight percent, and the balance being the carrier.