In this paper, tundish flux system for high Al TRIP steel’s slab casting was studied. Firstly the thermodynamical calculation of Al-SiO2 reaction was carried out. Secondly 27 groups of chemical compositions of tundish flux were designed using orthogonal method on the basis of thermodynamical results and slag phase diagram. Thirdly the melting point and the viscosity of each group of chemical composition were tested. Then, regression equations of melting point and viscosity were obtained according to experimental data as well as the analysis of the effect of single-factor and double-factors variation on the regression equations. Finally, the optimal design of tundish flux is w(CaO)/w(Al2O3)=1.1-1.2, w(SiO2)<5%, w(MgO)=4%-7%, w(CaF2)=7%-10% and w(Na2O)=5%, which has a relatively low melting point and viscosity, and its physicochemical performance is relatively stable.