Hot processing behavior of an ultra-high-strength Fe-Ni-Co-based maraging steel was studied in temperature range of 900-1200 ℃ and strain rate range of 0.001-10 s-1.Deformation processing parameters and optimum hot working window were characterized via flow stress analysis,constitutive equation construction,hot processing map calculation and microstructure evolution,respectively.Critical strain value for dynamic recrystallization was determined through theoretical mathematical differential method:the inflection point of θ-σ and-(e)θ/(e)σ-σ curves.It was found that the flow stress increased with the decrease in deformation temperature and increase in the strain rate.The power dissipation maps in the strain range of 0.1-0.6 were entirely similar with the tendency of contour lines which implied that strain had no strong effect on the dissipation maps.Nevertheless,the instability maps showed obvious strain sensitivity with increasing strain,which was ascribed to the flow localization and instability.The optimized hot processing window of the experimental steel was obtained as 1100-1200 ℃/0.001-1 s-1 and 1000-1100 ℃/0.001-0.1 s-1,with the efficiency range of 20-40％.Owing to high Mo content in the experimental steel,high dynamic activation energy,Q =439.311 kJ mo1-1,was achieved,indicating that dynamic recrystallization was difficult to occur in the hot deformation process,which was proved via microstructure analysis under different hot deformation conditions.