The invention discloses a method for preparing composite solid super acid. The method comprises the following steps of: rapidly dipping, depositing and loading TiO2 under the ultrasonic condition by using a molecular sieve as a carrier; washing and concentrating by using a ceramic membrane separation device without filtering and drying; directly loading SO4<2-> in the next step; and finally performing solid-liquid separation, drying and baking to obtain a composite solid super acid catalyst. The composite solid super acid catalyst synthesized by the method has the advantages of high acid strength, low production cost, simple flow, mild preparation condition, high speed and efficiency, is suitable for large-scale industrial production and has broad application prospect.