Abstract: Heat treatment experiments were conducted on Q345 R steel under different fire temperatures, insulation durations and cooling treatment, which could reflect the influence of fire on the material. Hence the mechanical properties and corresponding microstructure of the material were further evaluated. It is found that at the fire temperature of 20 ℃~649 ℃, the mechanical properties of the material can meet the requirements of application. If the fire temperature rises to 732 ℃~816 ℃, compared with the air cooling, water cooling can affect the mechanical properties of the material more. When the fire temperature is lower than 649 ℃, the microstructure of the material is ferrite and pearlite, however, when the temperature is higher than 649 ℃, the metallographic structure changes significantly. The grain size by water cooling is less uniform than that by air cooling. There is no obvious decarburization when the heat treatment temperature is lower than 649 ℃. However, the maximum depth of decarburization layer on the surface of the material will be approximately 0.20 mm when the temperature reaches 816 ℃. As the fire temperature increases, the surface color of the heat treatment becomes from reddish brown, to taupe, to dark red and to bright red at last.