Abstract: The mechanical properties and fracture toughness of high-strength structural steel Q420 were studied by tension tests and three-point bending tests.The results indicated that the temperature had significant influence on fracture toughness of structural steel and the fracture toughness reduced significantly with the decrease of temperature resulting in brittle fracture.The prestrain increased the yield strength and tensile strength of structural steel,but significantly reduced its plasticity and fracture toughness,and further increased the possibility of brittle fracture.The prestrain facilitated the brittle fracture due to the activation of the near crack tip stress fields because of the prestrain based on the FEA.Thus the effects of the temperature and the prestrain should be considered in the important engineering design,material selection,safety analysis and evaluation.