Prediction of stroke for precision straightening is core part of flexible automatic straightening machine. Straightening process is a nonlinear process, and affected by many factors. In this paper, in order to improve the straightening precision, an expert system is developed based on the previous research, which mainly consists of database for keeping data, expert program for self-study and procedures for application. This paper focuses on designing the expert system and application process. Humanization of the software interface has been designed for ordinary operators and researchers. And at last this system’s feasibility is verified by comparison between these two results, the performance of established expert system is excellent. It will be helpful to the development of automatic straightening machine’s control system.