In order to research the mechanical properties of 600 MPa grade high-strength steel bar after high temperature, the tensile strength test on HTRB600 high-strength steel bar after high temperature is conducted. The specimens are heated up to 20, 200, 300, 400, 500, 600, 700 and 800℃ using a electric furnace respectively, then the tensile test is carried out after the specimens are cooled in air and the yield strength, ultimate strenth, elastic modulus and stress-strain curve of the specimens are also measured and recorded simultaneously. The experimental results show that when the temperature is lower than 600℃, the stress-strain curve of the HTRB600 steel bar after high temperature has no obvious difference compared with that of the unheated specimens; When the temperature reaches 700℃, the yield strength and ultimate strength of the steel bar after high temperature reduce to 80% of those of the unheated specimens; When the temperature reaches 800℃, the yield strength of the steel bar after high temperature reduces to 60% of that of the unheated specimens whereas the ultimate strength s still maintaines at the same level to that of the 700℃ heated specimens. The influence of temperature on the elastic modulus of the HTRB600 steel bar after high temperature is also not ovbious. At last, the simplified calculation method for the stress-strain curve of HTRB600 steel bar after high temperature is proposed.