An experimental study was carried out to investigate the mechanical properties of domestic high strength steel Q550 at elevated temperatures. Steady-state tests were conducted on specimens at high temperatures ranged from 20 ℃ to 800 ℃. The experimental phenomena, mechanical properties and stress-strain curves at different temperatures were obtained. The tests results were compared with the reduction factors determined by current standards and by other researchers. The results showed that, the surface and fracture of the specimens at different temperatures were significantly different. When the temperature of specimen was higher than 300 ℃, the elastic modulus, yield strength and ultimate strength decreased with the increase of temperature, the linear elasticity and strain-hardening stage of the stress-strain curves shortened, and the descending stage tended to be gentle. The high temperature had little influence on elongation when the temperature was below 450 ℃. The current parameter models of high temperature mechanical properties were not applicable for Q550 steel. In order to obtain the mathematical models for mechanical properties of Q550, polynomial model and high temperature material model proposed by National Institute of Standards and Technology were employed.