<p>In shale oil and gas development, the orientation of horizontal wells often deviates from the minimum horizontal stress direction, leading to complex multi-cluster fracture propagation and reduced stimulation effectiveness. To clarify the underlying mechanisms, a three-dimensional flow–solid coupled numerical model for multi-cluster hydraulic fracturing in deviated horizontal wells was developed using the lattice method and validated against the analytical solution of a Penny-shaped fracture. Three quantitative indicators—average deflection angle, area standard deviation, and total fracture area—were introduced to describe fracture deflection, propagation uniformity, and stimulation scale, respectively. A reservoir stimulation efficiency index was further proposed to evaluate the overall fracturing performance. Results show that fracture propagation is governed by the coupling of in-situ stress anisotropy and inter-cluster stress interference: a large horizontal stress difference tends to induce “S-shaped” fractures, while strong inter-cluster interference promotes “bowl-shaped” fracture geometries. The controlling effects of geological and engineering parameters vary significantly: wellbore azimuth, cluster number, and fluid viscosity mainly affect fracture deflection; injection rate and cluster spacing dominate propagation uniformity; while cluster number and fluid viscosity exert the greatest influence on stimulation scale. Comprehensive evaluation indicates that reservoir stimulation efficiency is controlled sequentially by cluster number, fluid viscosity, injection rate, cluster spacing, wellbore azimuth, and horizontal stress difference. Under a fixed wellbore orientation, optimizing cluster spacing and fluid viscosity, targeting low-stress-difference zones, and properly reducing cluster number while increasing injection rate can significantly enhance fracturing performance. This study provides theoretical and practical insights for optimizing fracturing design in deviated horizontal wells.</p>