This study investigates the hot-deformation behavior and dynamic recrystallization (DRX) kinetics of 12Cr2Mo1V steel for heavy forgings of large hydrogenation reactors. Gleeble isothermal compression tests were performed at 950–1200 °C and strain rates of 0.01–5 s⁻¹ with friction correction to obtain reliable flow–stress data. A strain-compensated Arrhenius constitutive model was established to characterize the hot flow behavior over the entire strain range, with R = 0.9918 and AARE = 3.8826%. The dynamic material model was used to determine the optimal stable processing window of 1000–1200 °C and 0.01–0.1 s⁻¹. Microstructural analyses reveal that continuous dynamic recrystallization (CDRX) dominates grain evolution below 1100 °C, while discontinuous dynamic recrystallization (DDRX) prevails at higher temperatures. A mechanism-segmented DRX model with a 1100 °C transition threshold is proposed to precisely describe the temperature-dependent dual-DRX kinetics of the studied steel. The calibrated constitutive equation and segmented DRX sub-models were embedded into the DEFORM finite-element software for coupled thermo-mechanical–microstructural simulations, and the simulated microstructures agree well with experimental results. This work supports refined numerical simulation and process-parameter optimization of heavy reactor forgings, while providing a convenient and efficient modeling strategy for alloys with temperature-dependent dual-DRX mechanisms.