For the problems of lower yield strength and impact energy in high-temperature and high-strength special steel, it was found that the main cause was the local stress concentration in the material, which was caused by the microstructures such as gains with overly large sizes and aggregated distribution of large-sized Nb-rich phase. Through systematic microstructural characterization, the grain coarsening mechanisms and the dynamic recrystallization behavior were elucidated. Based the synergistic shear effect theory of compression-torsion composite deformation, a multi-directional shear stress field was introduced in uniaxial deformation system, establishing an strain path conducive to dynamic recrystallization successfully. A novel fine-grain strengthening process was developed through optimization of compression-torsion forming process parameters and heat treatment system. Experimental results demonstrate that the compression-torsion process with severe deformation can achieve the  simultaneous grain ultra-refinement (grain size is Grade 6.0) and the homogeneous dispersion of Nb-rich phases, resulting in enhanced mechanical properties with the yield strength of 941 MPa and the impact energy of 27.6 J.