Abstract: To improve mechanical property of the microalloyed steel, a novel “forging–annealing” cycle heat working was developed. Ti−Nb−V−B microalloyed steel was used as the experimental object to study the effect of “forging-annealing” cycle on the grain size and mechanical properties of Ti−Nb−V−B microalloyed steel. And then, TEM characterization and thermodynamic calculation were conducted to reveal the mechanism of grain refinement of Ti−Nb−V−B microalloyed steel. On this basis, its grain refinement model was constructed. The results show that the “forging-annealing” cycle heat treatment can significantly refine the grain size of Ti−Nb−V−B microalloyed steel. After the “three forging and three annealing” treatment, the grain size is refined from 2.5–4.0 levels before forging to 9.5–11.5 levels. The tensile strength (R_m), yield strength (R_el), elongation after fracture (A), reduction of area (Z), and Charpy impact energy (K_V) of the “three forging and three retreating” specimen have increased by 10.6%, 9.9%, 18.8%, 30.2%, and 135.0% respectively, compared to “one forging and one retreating”. he precipitation temperature of NbC, TiC, VC and Cr₂₃C₆ in Ti−Nb−V−B microalloyed steel are 1 202, 1 187, 897 and 672 ℃, respectively. The dissolution of VC and Cr₂₃C₆ occurres during the pre-forging heating and forging process, while annealing promotes the precipitation of the second phase, and the precipitation of NbC and TiC occurres during the “forging−annealing” cycle.The synergistic effect of austenite recrystallization before forging, forging recrystallization, and second-phase precipitation by annealing after forging is the key mechanism for obtaining fine grains in the Ti−Nb−V−B microalloyed steel.