The hot deformation behavior of the equiatomic FeCrNiMn high entropy alloy was studied through the isothermal compression at 900~1050 °C with strain rate of 0.001~1 s^-1. Results show that the initial microstructure is mainly composed of equiaxed grains with face-centered cubic structure and fine body-centered cubic phase particles. The flow curves exhibit the typical single stress peak type. The peak stress is decreased significantly with increasing the temperature and decreasing the strain rate. The constitutive model was developed based on the hyperbolic-sine law to predict the flow stress. The stress exponent and the apparent activation energy were calculated as 3.13 and 405 kJ/mol, respectively. The processing maps were obtained based on the dynamic material model at different strains. It is found that no instability zone appears in the processing maps at different strains, suggesting the excellent hot deformability of the alloy. The deformation microstructure is closely related to the power dissipation efficiency η. The recrystallization fraction is 17.6vol% when η is 28%, and it is increased to 37.5vol% when η is 38%. Two optimal hot working condition ranges can be identified according to the processing maps: 900~940 °C/strain rate of 10^-3~10^-1.3 s^-1 and 960~1050 °C/10^-3~10^-0.3 s^-1.