An on-line continuous casting slab surface quality prediction expert system is developed. The developing work is nearly completed and it may be the first system of the relating field in China. The main structure of the on-line continuous casting slab surface quality prediction expert system is that the slab surface quality is predicted online with expert system, and the system can alarm breakout in continuous casting with BP neural net and logic methods, based on the collected thermal couple data in mould and the process parameter transferred from the Continuous Casting machine server.