The dynamic recrystallization (DRX) behavior of austenite in reduced activation ferritic/martensitic steel was investigated by isothermal compression tests and constitutive modelling. The flow stress decreases with deformation temperature increasing and strain rate decreasing. The constitutive equations were constructed based on the analysis by Arrhenius equations. Based on the dynamic material model, the hot processing map was constructed. In the stability region, the power dissipation efficiency (η) increases with increasing strain, and the softening is dominated by DRX when η grows to a specific value. However, η decreases as strain increases further after the complete evolution of DRX, indicating coarsening of DRXed grains. As the strain increases, the processing stability region shifts to the low temperature and high strain rate region. A novel recrystallization kinetics model was developed by considering the characteristics of the recrystallization velocity with strain increasing. It can reflect the "slow-rapid-slow" growth trend of recrystallization volume fraction (XDRX). The relationship between XDRX and (average grain size of austenite) was constructed, which indicates that decreases with strain increasing after the onset of DRX. When XDRX is greater than 50%, the decrease rate of slows down until the steady-state is reached. Although strain increase further, the grain size remains constant. This suggests that the expected can be obtained by controlling XDRX during hot deformation.