Isothermal hot compression tests of as-cast high-Cr ultra-super-critical (USC) rotor steel with columnar grains perpendicular to the compression direction were carried out in the temperature range from 950 to 1250°C at strain rates ranging from 0.001 to 1 s^-1. The softening mechanism was dynamic recovery (DRV) at 950°C and the strain rate of 1 s^-1, whereas it was dynamic recrystallization (DRX) under the other conditions. A modified constitutive equation based on the Arrhenius model with strain compensation reasonably predicted the flow stress under various deformation conditions, and the activation energy was calculated to be 643.92 kJ·mol^-1. The critical stresses of dynamic recrystallization under different conditions were determined from the work-hardening rate (θ)–flow stress (σ) and -∂θ/∂σ–σ curves. The optimum processing parameters via analysis of the processing map and the softening mechanism were determined to be a deformation temperature range from 1100 to 1200°C and a strain-rate range from 0.001 to 0.08 s^-1, with a power dissipation efficiency η greater than 31%.