Abstract: Ultra-high strength steel (UHSS) fabricated via laser additive manufacturing (LAM) holds significant promise for applications in defense, aerospace, and other high-performance sectors. However, its response to high-impact loading remains insufficiently understood, particularly regarding the influence of energy density on its dynamic mechanical behavior. In this study, scanning electron microscopy, electron backscatter diffraction, and image recognition techniques were employed to investigate the microstructural variations of LAM-fabricated UHSS under different energy density conditions. The dynamic mechanical behavior of the material was characterized using a Split Hopkinson Pressure Bar system in combination with high-speed digital image correlation. The study reveals the spatiotemporal evolution of surface strain and crack formation, as well as the underlying dynamic fracture mechanisms. A clear correlation was established between the microstructures formed under varying energy densities and the resulting dynamic mechanical strength of the material. Results demonstrate that optimal material density is achieved at energy densities of 292 and 333 J/mm³. In contrast, energy densities exceeding 333 J/mm³ induce keyhole defects, compromising structural integrity. Dynamic performance is strongly dependent on material density, with peak impact resistance observed at 292 J/mm³—where strength is 8.4% to 17.6% higher than that at 500 J/mm³. At strain rates ≥ 2000 s⁻¹, the material reaches its strength limit at approximately 110 μs, with the initial crack appearing within 12 μs, followed by rapid failure. Conversely, at strain rates ≤ 1500 s⁻¹, only microcracks and adiabatic shear bands are detected. A transition in fracture surface morphology from ductile to brittle is observed with increasing strain rate. These findings offer critical insights into optimizing the dynamic mechanical properties of LAM-fabricated UHSS and provide a valuable foundation for its deployment in high-impact environments.