Intermediate heat exchanger (IHX), which transfers the heat generated in the reactor core to the secondary loop, is one of the key structural components of the very high-temperature gas-cooled reactor (VHTR). The Ni-based superalloy GH3128 has good high-temperature strength and so is a promising main structural material for the IHX. In this paper, the flow stress behaviors and the deformation microstructure of superalloy GH3128 were investigated by high-temperature compression tests conducted at various temperatures (950–1150 °C) and strain rates (0.001–10 s−1), and the processing maps were analyzed in order to establish the hot deformation constitutive model and obtain the optimum hot forming condition. The results show that (1) both flow stresses and peak flow stresses increase along with the increase of strain rate or decrease of temperature, (2) GH3128 has excellent hot workability, (3) the dynamic recovery (DRV) plays the dominant role during the dynamic softening process due to the high stack fault energy, and (4) the optimum hot forming condition of GH3128 should be defined in the temperature of 1150 °C and strain rate range of 0.01–0.056 s−1. This work contributes to the application of GH3128 alloy on IHX structure.