Monolithic stoppers play a very important role in continuous casting (short for CC). Sometimes, the stopper slag line is seriously eroded and broken, CC has to pause. The research shows that there are three main reasons which bring severe erosion of monolithic stopper slag line. Firstly, much Fe₂O₃ and MnO in slag react with carbon (C) in the stopper, C is gradually eroded and erosion resistance of the stopper greatly weakens. Secondly, F increase in tundish covering flux will quicken erosion of the stopper slag line. Thirdly, High LOI of tundish covering flux also makes erosion of the stopper slag line speed up. On one hand, CaCO₃ in tundish covering flux decomposes and brings CO₂; on the other hand, minerals with water in tundish covering flux decompose at high temperatures and bring water vapor. Then CO₂ and water vapor react with C in the stopper, C is gradually eroded, aggravating the erosion.