Pro/3 v1.1.1-the production system-type expert system shell based on Visual Prolog6.2 is introduced. The parameter rule function of inexact rules was corrected in the Pro/3. Its corrective code is given. The expert system about shopping and assessment of a current transformer was implemented by researching the Pro/3. The goal of the expert system is that it gives a degree of suitability (membership grade) description for preference current transformer to a user by analysing a request and preference of the user for the current transformer brand, type, price etc. and deriving sentences for making decision. The sentences and application of rules are partly illustrated in the paper. Finally, the result of practice verifies the validity and the practicability of the expert system.