Balanced propagation of multiple hydraulic fractures within a stage is difficult to achieve during staged fracturing of horizontal wells in shale reservoirs, and the effectiveness of reservoir stimulation is therefore limited. In this study, a fluid–solid coupling model for synchronous propagation of multicluster hydraulic fractures was established. A fracture uniformity coefficient was introduced to quantitatively characterize the degree of balanced fracture propagation. A nine-factor, five-level orthogonal design was adopted. The effects of engineering and geological parameters on fracture propagation balance were systematically analyzed. The results show that the fracture uniformity coefficient ranges from 0.036 to 0.537 under 125 parameter combinations. The influencing factors are ranked as cluster number, cluster spacing, injection rate, fracturing fluid viscosity, fracture toughness, horizontal stress difference, Poisson’s ratio, fracture height, and Young’s modulus. As the cluster number increases from 3 to 7, the average fracture uniformity coefficient increases from 0.194 to 0.402. The propagation non-uniformity is therefore significantly aggravated. As the cluster spacing increases from 5 m to 13 m, the average fracture uniformity coefficient decreases from 0.380 to 0.214. As the injection rate increases from 8 m3/min to 16 m3/min, the average fracture uniformity coefficient decreases from 0.364 to 0.222. Balanced fracture propagation is thus significantly improved. Stronger effects are exerted by engineering parameters than by geological parameters. Balanced fracture propagation can be effectively promoted and the stimulated reservoir volume can be enlarged through proper optimization of cluster number, cluster spacing, and injection rate. A theoretical basis is therefore provided for fracturing parameter optimization and efficient reservoir stimulation of horizontal wells in shale reservoirs.