Voronoi tessellation is popular in porous structure design, known for its excellent impact resistance and energy absorption. However, the mechanical properties of Voronoi porous structures are apt to be affected by the gradient of pore size. In this study, the influence of gradient parameters on the mechanical properties of Voronoi porous structures is investigated to achieve the mechanical property enhancement. Three types of gradient Voronoi porous structures with varying gradient spans (G–2, G–3, G–4) were designed by controlling the distribution of seeds in the Voronoi pattern. The gradient Voronoi porous samples with 70% porosity were fabricated using selective laser melting (SLM) technology. The influence of gradient parameters on the mechanical properties of gradient Voronoi porous structures was investigated by experiments and finite element simulation, and the deformation mechanisms within their intrinsic structures were analyzed. The results demonstrate that the yield strength of the gradient Voronoi porous structures improves with increasing gradient span. Moreover, among these structure, G–3 exhibits superior energy-absorption ability as a result of possessing higher flow stresses during both strain platform and densification stage due to its moderate gradient span. Smaller gradient spans lead to localized densification region formation within the structure; small pores and high pore densities cause more drastic deformation in internal support structure, thereby enhancing densification behavior.