Abstract: To meet the demand for ultra-high-strength steels for next-generation deep-sea pressure hulls, the effects of prior austenite deformation (ausforming)o n the martensitic microstructure and mechanical properties of HY180 steel were investigated. A Ni-Co-based HY180 steel was used as the experimental material. The specimens were heated to 900 ℃ and held for 300 s in a thermomechanical simulator, then cooled to 600, 700, and 800 ℃, respectively, and subjected to single-pass hot compression with a reduction of 50%. Specimens directly quenched from 900 ℃ were prepared as the reference. The microstructures and mechanical properties were characterized using optical microscopy, scanning electron microscopy, electron backscatter diffraction, microhardness testing, and sub-size tensile testing. The results show that decreasing the deformation temperature increases the flow stress and work-hardening capacity. Although the microstructure remains predominantly lath martensite after hot deformation, both the prior austenite grains and martensitic microstructure are significantly refined. The prior austenite grain size decreases from 7.32 μm to approximately 5.00 μm, while the effective martensite grain size decreases from 1.06 μm to 0.76- 0.87 μm. The increased kernel average misorientation (KAM)value indicates enhanced local orientation gradients and dislocation storage. For the specimen deformed at 600 ℃, the microhardness increases from 449HV to 507HV, the yield strength increases from 1 192 MPa to 1 368 MPa, and the ultimate tensile strength increases from 1 363 MPa to 1 451 MPa, while no evident reduction in total elongation is observed. These results demonstrate that, without altering the chemical composition, prior austenite deformation can effectively enhance the strength of HY180 steel through microstructural refinement and increased dislocation storage. This study provides an experimental basis for the thermomechanical microstructure control of Ni-Co-based martensitic ultra-high-strength steels. Further investigations involving multiple deformation passes, smaller reductions per pass, and continuous cooling conditions are required to evaluate the engineering applicability of this processing route.