Abstract: The mechanical properties of a certain SM490 steel plate were found to be unqualified. The reasons for the unqualified mechanical properties of the steel were analyzed using macroscopic observation, metallographic examination and process investigation methods, and improvement measures were proposed. The results show that the unreasonable rolling temperature control process led to the appearance of Widmannstatten structure and network ferrite in the steel, resulting in a decrease in the plasticity of the metal material. There were obvious processing defects such as grooves and steps in the tensile test sample. During the tensile test, stress concentration occurred at the defect site, ultimately led to the mechanical properties of the sample being unqualified.