In order to study the post-fire mechanical properties of high strength Q960 steel， the steady state tensile test on high strength Q960 steel specimens after exposure to 300-900 ℃ was carried out. The stress-strain curves， elastic modulus， yield strength and ultimate strength were determind after air cooling and water cooling. The results show that 600 ℃ is the critical temperature of strength loss of high strength Q960 steel. The test results are compared with properties of ordinary Q235， Q345 steels and high strength Q460， Q690， S960 steels. The comparison shows that the degradation degree of mechanical properties after exposure to high temperature varies among different steels. Based on the test results， new fitting equations for evaluating the post-fire mechanical property reduction factors of high strength Q960 steel are proposed. The fitting results are in good agreement with the experimental results.