In this paper, the microstructure and properties construction steel bars under various quenching and tempering temperature are investigated. Tensile tests show that the maximum tensile strength is1489MPa, elongation 11%, quenching temperature and tempering temperature have effects on the strength and plasticity of the samples, but the latter effect is more pronounced than the former. Steel bars have superior delayed fracture resistance between 390 ~420°C. The microstructure of steel bar is mainly composed of fine tempered sorbite (troostite) with carbide distributed along the lath martensite boundaries. Thus, we draw the conclusion thermal refining is an effective way to improve mechanical properties and delayed fracture resistance of high strength construction steel bars.