Abstract: To investigate into the deformation of the Mach stem in a strong shock reflection, the flow field near the intersection point between slip line and wedge surface was theoretically analyzed, and a transition criterion for Mach stem deformation to disappear was derived, which was solved with the shock polar method. Processes of Mach stem deformation were described, and effects of ratio of specific heats and Mach number on conditions for Mach stem deformation to disappear, which were compared with those in presence of terminal double Mach reflections, were investigated. The results show that the disappearance of Mach stem deformation is a reflection phenomenon of shock waves in mediums with low ratios of specific heats. The disappearance of Mach stem protrusion leads to a new configuration of double Mach reflection with a straight Mach stem and no wall jet.