Abstract: In order to ensure fire-resistant property of construction steel under the condition of the least Mo,Nb addition,effects of Mo and Nb content on microstructure and properties of the fire-resistant construction steel,specially on high temperature strength were investigated.The results show that the room temperature yield strength,tensile strength and yield strength at 600℃ increased with increasing Mo,but toughness decreased,the quantities of bainite increased simultaneously.At high temperatures the stable and tiny carbides could well improve high temperature strength of steels.Co-addition of Mo and Nb could achieve better effect of high temperature strength,adding 0.25% Mo and 0.021% Nb could meet fire-resistant demand of construction steel,and better economical efficiency could be obtained.