The invention provides a catalyst system for producing high-cleanness diesel oil, which comprises mixed acid of concentrated sulfuric acid and concentrated phosphoric acid, and a mixture dispersed in the mixed acid and consisting of aluminum oxide, magnesium oxide and silicon dioxide. The invention also provides a method for producing the high-cleanness diesel oil, which comprises a step of pre-treating raw materials for the catalyst system of the invention. According to the catalyst system and the method, the high-cleanness diesel oil can be produced. The high-cleanness diesel oil prepared by the method can form synthetic diesel oil meeting the requirement of international GB252-2000 standard through blending. Moreover, the process is relatively simple, does not consume energy, and has low unit production cost.