Abstract: Core analysis from hydraulic fracturing field tests indicates that proppant has difficulty entering sub-millimeter-scale secondary fractures located far from the wellbore, resulting in a significant portion of the fracture network remaining unpropped. Therefore, elucidating the diversion and entry mechanisms of proppant and enhancing the effective propped volume of the fracture network are critical for improving the productivity of fractured wells in unconventional oil and gas reservoirs. To address this issue, a proppant transport model was established based on the computational fluid dynamics-discrete element method (CFD-DEM), and its accuracy was thoroughly validated against stereoscopic particle image velocimetry (SPIV) experimental measurements. On this basis, the effects of fracture width, intrinsic material properties, and operational parameters on proppant diversion and entry behavior were systematically investigated. The results demonstrate that the ability of proppant to divert from the primary fracture into secondary fractures is primarily governed by the matching relationship between proppant particle size and the widths of the two fracture levels. Through the analysis of fluid-particle flow partitioning, it is revealed that proppant entry behavior is intrinsically constrained by a “weakest-link effect,” that is, a single unfavorable factor can significantly suppress proppant entry, causing the particle flow ratio to fall notably below the fluid flow ratio. Accordingly, regime maps were established with particle size, fracture width, Reynolds number (Re), and Stokes number (St) as key parameters, based on which a criterion for proppant entry was proposed. It is demonstrated that when the ratio of particle size to secondary fracture width exceeds 0.4, proppant can hardly enter the fracture, whereas when this ratio falls below 0.2, the particle flow fraction can exceed 50%. Furthermore, to achieve effective proppant placement within the fracture network, a parameter optimization method guided by the “weakest-link effect” was proposed based on the matching relationship between fluid and particle flow partitioning. The findings of this study provide theoretical support and technical guidance for the efficient and sustainable development of unconventional oil and gas resources.