The flow behavior of a medium entropy alloy with a nominal composition of Fe_0.25Cr_0.25Ni_0.25Mn_0.25 was analyzed by isothermal compression performed in the temperature range of 900~1050 °C and strain rate range of 1~0.001 s^-1. The results show that the hot deformation is predominated by dynamic recrystallization, so that the flow curves exhibit a single-peak shape as those of other alloys with low stacking-fault energy. Particular emphasis was paid to develop a simple constitutive model which can describe the entire deformation history. For this purpose, the work-hardening behavior as well as the dynamic softening regime were analyzed. With the aid of Kocks-Mecking plots, it is found that the hardening rate of the present alloy is linearly decreased with stress in the work-hardening stage, and hence the stress-strain behavior can be described by the conventional dislocation density-based model. Meanwhile, the softening regime, which is caused by dynamic recrystallization, can be modelled by the classic JMAK equation. Besides, the model is further modified to reduce the number of parameters and simplify the regression analysis. The proposed semi-physical based model can not only accurately predict the stress-strain behavior to strain levels outside the experimental strain range, but can also be promoted to other alloys with low stacking-fault energy.