Abstract: In order to analyze the reason for the low reduction of area of 35CrMnSi low alloy high strength steel in tensile test, the composition, residual hydrogen, microstructure, fracture morphology and inclusions of the tensile sample were observed and analyzed by direct reading spectrometer, oxygen-nitrogen hydrogen analyzer, metallographic microscope, scanning electron microscope and energy spectrometer. The results show that there was a certain amount of hydrogen in the tensile specimen of 35CrMnSi steel. Under the combined action of tensile stress, the “fish-eye” defect was the main reason for the low reduction of area of the tensile specimen.