To overcome the limitations of poor room-temperature ductility and significant anisotropy in AZ91 magnesium alloy, a constant-strain-rate backward extrusion (CSR-BE) process was introduced to enhance deformation homogeneity through synergistic regulation of die curvature and strain rate. The methodology employs axisymmetric slip-line theory, combined with volume constancy principles, to derive a die profile equation that enables precise control of strain rates throughout deformation. Comprehensive DEFORM-3D simulations reveal that CSR-BE achieves a 9.3% reduction in extrusion force compared to conventional backward extrusion, accompanied by substantial improvements in deformation uniformity: a 99.96% decrease in flow velocity variance, an 85.2% reduction in stress field variation, and an 81.6% mitigation of temperature distribution fluctuations. Mechanistic analysis demonstrates that the optimized die geometry shifts the material flow dominance from radial shear to axial stretching, characterized by a decrease in the slip-line orientation angle from 36.27° at the inlet to 11.3° at the outlet. The engineered hydrostatic pressure gradient effectively alleviates localized stress concentrations, eliminates friction-induced dead zones, and suppresses strain-rate variations. Quantitative stress and flow-rate calculations confirm that this approach fundamentally addresses the microstructural heterogeneity inherent to traditional extrusion methods. CSR-BE process establishes a theoretically grounded manufacturing strategy for producing high-performance magnesium alloy rods with enhanced isotropy, demonstrating significant potential for industrial-scale applications through its combined efficiency and microstructural control advantages.