Abstract: Continues cooling phase transformation behavior of the Mo-Cr-Nb fire-resistant test steel under simulated cooling process were studied by Gleeble, TEM and SEM, the refractory mechanism of fire-resistant rebar were analyzed systematically. The result indicates the ferrite fraction increases from 14.5% to 36.1%, the strength decreases and plasticity increases with the increase of final cooling temperature from 700 ℃to 1000 ℃; the hard phase bainite and martensite fraction increases and occupies the main status gradually, the strength increases and plasticity decreases with the increase of cooling rate from 0.5 ℃/s to 20 ℃/s.At 800 ℃final cooling temperature and 1 ℃/s cooling rate, the proportion and size of ferrite of the tested sample is 28.2% and 12.3 μm, respectively, and the strength and plastic match optimally. At 600 ℃heating and preservation for 1h, large upper bainite of test steel decomposes into high-reliability alloy carbide cluster,and the fine precipitates coalesce to form Mo containing composite precipitates, the interaction of both ensure the fire-resistant property of test steel.