Abstract: Tensile testing of HTRB600 steel bars under high temperature was conducted to determine the mechanical properties of the steel under high temperature. The elastic modulus, proportional limit, yield strength, ultimate strength, and stress-strain curves of steel bars at 20, 200, 300, 400, 500, 600, 700, and 800℃ were obtained. The experimental results show that the elastic modulus, proportional limit, yield strength, and ultimate strength of the bars obviously decrease with increasing temperature. The yield strength, ultimate strength, proportional limit, and elastic modulus of the steel bars at 500 and 800℃ decrease to less than 50% and 10%, respectively, of those of the sample at room temperature. The stress-strain curve of the steel bars smoothens as the temperature increases, and the yield terrace disappears at 200℃. The degree of decrease in yield strength and ultimate strength of the 600 MPa steel bar is obviously larger than that of the < 500 MPa steel bar. Finally, a simplified calculation method to determine the stress-strain curve of HTRB600 steel bars under high temperature was proposed.