Abstract: To control slag entrapment, a new method was put forward to calculate the level fluctuation index, i.e. F value, which was defined by other researchers. F value could easily be calculated based on the results of numerical simulation of slab continuous casting mold flow. The relationship between F value and level fluctuation in the mold as well as operational technological parameters were also studied. It is shown that using the data of numerical simulation of slab continuous casting mold flow can easily calculate F value. By adjusting operational technological parameters, the range of F value can be controlled in 3 -5, and the surface fluctuation in ± (3-5) mm, as a result, slag entrapment can be avoided.