<p>Radial borehole fracturing, integrating ultra-short radius radial drilling with hydraulic fracturing, shows promise for enhancing stimulation efficiency in conglomerate reservoirs. This study investigates the fracture propagation of radial borehole fracturing in conglomerate through true triaxial laboratory experiments using artificial conglomerate samples (300 mm × 300 mm × 300 mm). An orthogonal experimental design is applied to systematically evaluate the effects of gravel parameters, radial borehole parameters, geological parameters, and engineering parameters, with quantitative indices assessing fracturing effect. The results reveal four typical fracture morphologies—double-wing, slight-deflection, Y-shaped, and T-shaped—with multi-branch fractures occurring in over 50% of the samples. Fracture propagation is jointly governed by the in-situ stress field, radial boreholes, and gravel distribution. The synergistic interaction between the radial borehole and the weak interface at the gravel–matrix boundary reduces the dominant influence of the in-situ stress field, thereby extending fracture propagation along the radial borehole axis and promoting the development of complex fracture networks. Azimuth and horizontal stress difference are identified as the primary factors controlling fracture initiation and radial guidance, while fluid viscosity and pump rate mainly influence fracture complexity and breakdown pressure. Lower azimuths and stress differences improve fracture guidance along the borehole axis, whereas high-viscosity fluids enhance gravel penetration and increase breakdown pressure. These findings provide valuable theoretical insights for optimizing radial borehole fracturing in conglomerate formations.</p>