Abstract: The effects of tempering temperature on the tensile performance, 0℃ and-20℃ impact mechanical properties and micromorphology of hot rolled steel were studied, and the mechanism of the heat treatment was analyzed. The results showed that when the tempering temperature was below 450℃, the test steel had high strength and plasticity, good impact property and low yield ratio, which could meet the requirements of 690 MPa high strength and toughness aseismatic steel; that with the increasing of tempering temperature, the structure of the steel was still bainite, but the slab bainite coarsened and grew, the width of bainite slat kept constantly increasing, the boundary of the slab got gradually blurred and eventually turned into massive tissue; that the dislocation density decreased with the rising of the tempering temperature, and the bulk M/A islands gradually evolved from continuous distribution to chain like structure distributed among bainite slates, so that the dislocation strengthening effect decreased.