Abstract: This research focused on the influence of existing nozzles arrangement at the straightening zones of a caster on the cooling effect of slabs. Through a series of tests for combined nozzles on the nozzle automatic testing platform, the relationship between the spray overlap degree of adjacent nozzles and the uniformity of water distribution in the slab width direction was analyzed from three aspects including nozzle flowrate, injection height and hydraulic pressure. Based on the above research results, an optimization scheme was put forward for the secondary cooling system. Experiment results show that the internal quality of slabs improves significantly after optimization. Transverse corner cracks in slabs are almost eliminated, the center segregation level reduces from B 1.0 to C 1.5, and the ratio of equiaxed grains increases by 4.1%.