Ultra-high strength stainless steel possesses excellent mechanical properties, processing performance and corrosion resistance, and can usually be used to manufacture key devices in the aerospace, aviation and marine fields. However, there is still a lack of documentation for the reasonable design of hot processing maps of ultra-high strength stainless steel since research on hot processing technology for alloys is insufficient and valid technology for processing ultra-high strength stainless steel is lacked. In this work, USS122G ultrahigh-strength stainless steel, which has excellent performance and application potential, was chosen as the subjective alloy. The isothermal compression deformation behavior of the alloy was studied via a Gleeble-3500 thermal simulation experimental machine. The corresponding deformation (strain), deformation temperature and strain rate are approximately 60% (0.9), 930~1 130 ℃ and 10-2~101 s-1, respectively. On this basis, an Arrhenius-type constitutive model of the alloy was established and modified. Furthermore, a hot working diagram based on the Murty instability criterion was obtained. The results show that the flow stress of the alloy decreases with increasing deformation temperature (or decreasing strain rate). The flow stress of the alloy with a strain of 0.9 can be well predicted by the present Arrhenius-type constitutive equation. Moreover, the suitable strain rates for the alloy are 0.01~0.28 s-1, 0.1~0.28 s-1 and 0.01~0.81 s-1 for temperatures of 930~975 ℃, 975~ 1 050 ℃ and 1 050~1 130 ℃, respectively, according to the present hot working diagram.