SLM-fabricated 17-4PH stainless steel exhibits both high surface roughness and high hardness, making it difficult for traditional mechanical polishing to simultaneously maintain machining accuracy and surface integrity on complex curved surfaces. Ultraviolet femtosecond laser, leveraging its "cold" ablation mechanism, has emerged as a highly promising precision finishing process. However, the fundamental ablation data and morphological evolution characteristics of this specific material remain unclear. In this work, single-pulse ablation experiments were conducted using an ultraviolet femtosecond laser with a wavelength of 345 nm and a pulse duration of 300 fs, and the single-pulse ablation threshold of the material was determined to be 0.13 J/cm2 for the first time. The results reveal that with the increase of peak laser fluence (1.68–4.21 J/cm2), the ablation craters evolve from periodic ripple structures covered by nanoparticles into a composite structure with agglomerated particle protrusions in the center and residual ripples at the periphery, and the ablation mechanism transforms from mild phase explosion to intense phase explosion. This study provides key parameters and technical support for ultraviolet femtosecond laser finishing of additively manufactured metal components.