In order to solve the difficulties in the fault diagnosis on the hydraulic system of press, through analyzing the work principle of the hydraulic system and the common faults of press, the fault diagnosis expert system on the hydraulic system of press based on CBRFAT was proposed. The basic knowledge of expert system and the reasoning process of CLIPS were introduced. Based on CLIPS, knowledge representation of hydraulic fault cases and the methods of knowledge represention and rulebased reasoning on fault tree were illustrated. Accomplishment of the fault reasoning mechanism in the fault diagnosis expert system was discussed finally. The experiment shows that the system can realize the fault diagnosis on the hydraulic system of press well.