Abstract: Based on the Taigang 409 L steel casting production process and the parameter of the slab continuous cast¬er machine, influence of submerged nozzle structure on the molten steel flow behavior in the mould and the surface slag entrapment defect of [200 mm x (1060 〜1240 mm) ] casting slab are studied by means of water model experiment and industrial experiment. The results show that the nozzle structure currently in use ( side hole 48 mm x 70 mm, nozzle up dip 15° ) causes severe fluctuation of the liquid lever in mold and heavy entrapment slag on the surface of casting slab. With the condition of no-change nozzle structure, the problem of slag entrapment is impossible to be solved by using nozzle up dip 5° or 10°. The nozzle with side hole size 32 mm x52 mm can effectively solve the slag entrapment on steel409 L surface at small section [200 mm x (900 - 1100 mm) ] or low casting speed (0. 7 ~0. 9 m/min). The nozzle with inner diameter Φ60 mm the average wave height is between 3. 5 mm to 4. 5 mm and entrapment slag decreases from original 36.5% to 0.8% ,the submerged nozzle not only can be applied for present section [200 mm x (900 ~ 1320 mm) ] and casting speed (0.7 ~ 1.1 m/min) but also the casting slab quality can be improved.