The invention relates to synthesis of polystyrolsulfon acid and its application. Recovered polystyrene is used as a raw material and is purified by ethyl acetate dissolution; and fuming sulphuric acid is used for sulfonation so as to prepare polystyrolsulfon acid which is used as a catalyst to be applied in catalyzing an esterification reaction between polyol and carboxylic acid. Research results show that, in comparison with benzenesulfonic acid, the catalyst has advantages of low production cost, high esterification rate, convenient recovery, simple product aftertreatment technology, high viscosity index of an esterification product and the like.