The behavior evolvement of Mg-9Y-1MM-0.6Zr (WE91) alloy during hot deformation process was discussed. The flow stress behavior of magnesium alloy over the strain rate range of 0.001 to 1 s-1 and the temperature range of 653 to 773 K was studied on a Gleeble-1500D hot simulator under the maximum deformation degree of 60%. The experimental results showed that the relationship between stress and strain was obviously affected by the strain rate and deformation temperature and the flow curves observed were typical of the occurrence of dynamic recrystallization. The flow stress of WE91 magnesium alloy during high temperature deformation could be represented by the Zener-Hollomon parameter in the hyperbolic Arrhenius-type equation. The average deformation activation energy Q and strain coefficient n were 224.11 kJ/mol and 3.08 by calculation, respectively. The processing maps were calculated and analyzed according to the dynamic materials model. The map at strain of 0.916 exhibited three domains with peak efficiency of 49%, 44% and 42%, respectively. It was found that the alloy could be extruded at 693 K with mechanical properties of σ0.2=240 MPa, σb=315 MPa and δ=15.5%.