Abstract: To improve the internal quality of continuous casting billet, a new approach to quality analysis and optimum control is proposed on FTA Basis. The interrelation between different hypotheses of the formation mechanism of internal cracks is studied to find out the root causes. According to the features of FTA and expertise, an FTA model of internal cracks is developed, then the software VC++ 6.0 is introduced to form the visual-FTA system, which is available to generate and analyze the fault tree automatically. Applying the analysis system to the steel Q235 supplied by a steel-maker, the qualitative/quantitative results basically conform with the on-line statistical ones, then an optimum control scheme is proposed to improve the billet quality.