The paper studied the effect of high temperature characteristics of mold powder on the slab quality , finding that there is certain relationship between the surface quality of the slab and the carbon equivalent in cast steel ∑ (C), the viscosity of flux (η) and the square of casting speed (v2c). If the value of ηv2c / ∑ C increases, the surface defects of slab appear likely increase. If the value of ηv2c / ∑ C controlled in 3.1, the first pass rate of the slab increased significantly. Additionally if flux is long slag (temperature different of the melting zone △T ≥ 70 ℃),and melts faster (low than 1300 ℃,V ≤ 60 s,), greater risk of defects is.