Abstract: The impact absorbed energy at −50 ℃ of a certain batch of S355NL steel flange forgings used in wind power from a factory were low and fluctuated significantly. The cause of poor impact toughness at low temperatures was analyzed by comparing the mechanical properties, fracture morphology, and microstructure of qualified and unqualified flange forgings and combining with the production process. The results show that the impact fracture of unqualified flange forgings was characterized by brittle fracture, the fiber section rate was almost 0, and the crack propagation path was relatively straight during the impact process. The impact fracture of qualified flange forgings was characterized by a mixture of ductile and brittle fracture morphology, the fiber section rate was 60%, and the crack propagation path was more tortuous. High normalizing temperature and long heating time in the furnace resulted in coarse grains and uneven grain size distribution, that was the main reason for poor temperature impact toughness of S355NL steel flange forgings. Uneven distribution of pearlite in the structure was another reason for the reduction of toughness of flange forgings.