<p>3D sand printing (3DSP) technique is one of the most effective methods for fabricating physical model specimens. However, its limitations in the low strength and stiffness of printed specimens constrain its application in rock mechanics testing. To this end, this study proposes a novel postprocessing approach to enhance the mechanical properties of 3D sand-printed specimens through phenolic resin vacuum infiltration combined with high-temperature curing. Brazilian splitting tests, digital image correlation (DIC), scanning electron microscopy (SEM), and fractal geometry are employed to analyze the mechanical behavior and microstructure of printed specimens. Compared to untreated specimens, the tensile strength of treated specimens increases from 1.39&#xa0;MPa to 6.40&#xa0;MPa and the brittleness indices <i>B</i>₁, <i>B</i>₂, <i>B</i>₃ and <i>B</i>₄ increase from 5.14, 0.67, 4.27 and 4.97 to 17.95, 0.89, 60.65, and 367.65. The DIC analysis reveals that the yielding of treated specimens becomes insignificant, and the relative normal displacement of the failure surface increases from − 0.25&#xa0;mm to − 0.39&#xa0;mm. SEM and fractal dimension analysis confirm that phenolic resin fills pores and strengthens interparticle bonding, resulting in a denser microstructure with the lowest fractal dimension. The tensile strength has a negative relationship with the fractal dimension. The findings in this study provide a new pathway for fabricating physical model specimens.</p>