The flow behaviour and microstructure characteristics of a ferritic stainless steel were investigated using plain strain compression test on a Gleeble 3500 thermo-mechanical test simulator with a hydrawedge system in the temperature range of 850–1100 °C and strain rate range of 0.1–50 s−1. The phenomenological constitutive model and the relationship between the Zener–Hollomon (Z) parameter and flow stress were established. The results reveal that the flow softening phenomenon occurs at high strain rate, which is caused by the coupling effect of the adiabatic heating and dynamic recrystallisation (DRX). New grains nucleate preferentially at the original grain boundaries by strain-induced grain boundary migration. With an increase of temperature or strain rate, a part of new grains form in the interior of deformed grains. The DRX grain size and fraction increase with the increase of temperature, however, exhibit a non-linear relationship with strain rate.