The mold is one of the core components of steelmaking, and its flow field distribution will directly affect the quality of the casting slab. A three-dimensional nozzle model is built in this work, and fluid simulation is carried out to investigate the influence of the casting speed, immersion depth, slab thickness, and width on flow behavior in the mold. This model combined simulation with real conditions. The casting speed, immersion depth, slab width, and slab thickness are the actual process parameters used in the steel factory. The results show that when the casting speed increases from 0.6 to 1.0 m/min, the strike positions of the narrow surface are 0.439, 0.476, and 0.480 m below the liquid level, respectively. When the immersion depth increases from 180 to 220 mm, the impact depth of the stream at the exit of the nozzle side hole moves down, the lower recirculation zone moves to the centre and bottom of the slab, and the upper recirculation zone moves downward. When the slab thickness increases, the strike locations of the narrow face move down. Further, when the slab width increases, the distance of the strike location from the strike position increases first and then decreases. From the fluid results, the process parameters can be optimized by considering the strike location and the change of the surface turbulent kinetic energy. The model proposed in this work provides a theoretical guidance and optimization for the nozzle.