The number of steel-frame buildings is increasing as a result of development of social economy. However, the fire-resistant property of steel-frame buildings is much weaker than that of brick-frame buildings and RC-frame buildings. In response to such demand, fire-resistant steel has been developed since the last two decades. Mo is one of the most effective strengthening elements for the high-temperature strength of steels. With the increase of the Mo content in steels, there is a dramatic increase in costs which is unacceptable for the cost-sensitive construction area. Therefore, a fire-resistant steel with a low Mo content is highly desired. Two fire-resistant steels with low Mo content (<0.3%) were designed in this paper. The interdependence of microstructure, properties and temperature was studied and analyzed in hot-rolled testing steels. The results show that the low-Mo fire-resistant steels have improved elevated temperature strength (The yield strength of Nb content steel is 240MPa, which can met the requirements of grade Q345 fire-resistant steel), low yield ratio (<0.6) and good welding performance (Ceq≈0.40). The bainite and fine grains are mainly beneficial to the elevated temperature UTS. Nb is an alloying element effective in increasing elevated temperature strength (especially for YS) as well as Mo.