Based on the two-dimensional mismatch theory， the relative nucleation phases γ-Fe and α-Fe on NbC were calculated. The precipitation strengthening， fine grain strengthening， cooling control effect， microstructure and mechanical properties of microalloyed steel containing niobium were studied by means of scanning electron microscope（SEM）， transmission electron microscope （TEM）， optical microscope （OM）， dynamic thermal simulation transformation instrument and mechanical properties test. The results show that the effect of α-Fe and NbC heterogeneous nucleation is better， but the effect of γ-Fe and NbC heterogeneous nucleation is not ideal； in the continuous cooling process， the critical temperature of pearlite transformation is 1.5 ℃/s， and the critical temperature of bainite transformation is 5.0 ℃/s； the Nb containing HRB500 can effectively refine the grain size and improve the mechanical properties of microalloyed steel through appropriate controlled cooling process.