Abstract: Tensile tests of HAZ in Q345 FRE fire-resistant steel at 600 ℃ under conditions of different peak temperatures in single pass, different peak temperatures in multiple passes and differentt_8/5 times were carried out on Gleeble-3500 tester by thermal simulation method. The high temperature tensile properties, secondary high temperature tensile properties and their variations in the heat affected zone under different conditions were studied. The results showed that in the HAZs of single pass and double pass, the incomplete recrystallization zone at 870 ℃peak temperature was simulated and its high temperature yield strength and tensile strength at 600 ℃ were the lowest. The coarse-grained zone at 1 320 ℃ peak temperature was simulated and its high temperature yield strength and tensile strength at 600 ℃ were the highest and higher than those of parent metal. The effect of welding heat input（t_8/5） on high temperature yield strength and tensile strength at 600 ℃ was not obvious the incomplete recrystallization zone at 870 ℃ simulation peak temperature. The strength of the incomplete crystallization zone after secondary overheating by thermal simulation of did not decrease at t_8/5≤80 s.