This paper presents a comparative analysis of the mechanical properties of Q355, Q460, and Q690 steels during the cooling stages, based on experimental results. The results show that when the peak heating temperature within 700 ℃, the reduction factors of Young’s modulus for Q460 and Q690 steel at the fire-cooling stage are similar. The differences in reduction factor of yield strength f_0.2 among the three kinds of steels are large, while they are small in that of yield strength f_0.2 and ultimate strength. At peak heating temperatures of 800°C or 900°C, the differences in the reduction factors for Young"s modulus and yield strength between Q355, Q460, and Q690 steels at the cooling stage are more pronounced, while the differences in ultimate strength reduction factors are smaller. In addition, the high temperature mechanical properties of steel recommended by the existing specification are compared with the mechanical properties of the three kinds of steel at the fire-cooling stage. The results show that the reduction factors proposed in the existing specifications cannot predict the mechanical properties of steel at fire-cooling stage well.