Abstract: For the phenomenon of low-temperature impact toughness disqualification of S355NL steel, the steel plates were analyzed from the aspects of chemical composition, mechanical property, microstructure, inclusions, fracture and so on. The main parameters of the smelting, continuous casting, the rolling process were discussed with an emphasis. The results show that:the non-uniform grain size and coarse impurity particles were the main reasons leading to the low impact toughness of S355NL steel. By optimizing the smelting process to control the chemical compositions and inclusions, and optimizing the rolling process parameters to ensure the grain size of the steel plates, the qualified rate of the impact toughness of the S355NL steel was effectively improved.