Abstract: The radial inlet duct of a heavy-duty gas turbine results in additional losses and inlet distortion, thereby affecting the efficiency and stability of the compressor and subsequently impacting the efficiency of the gas turbine. Currently, the design of radial inlet ducts is based solely on empirical data, and a systematic parametric design method for such ducts has not yet been developed. Furthermore, the recommended range of values for each key dimensionless parameter is unavailable. Based on volute design theory, this paper simplifies the design of radial inlet duct and investigates its internal flow mechanisms using numerical simulations. The study identifies that the primary sources of flow loss within the inlet volute are the shear vortices beside the guide cone, the wake of struts, and the boundary layer in the contraction. Based on these observations, a parametric design method is proposed to analyze the effects of geometric structures on total pressure loss and flow distortion. The results indicate that the inlet diameter of the contraction, the diameter of the cone, and the cone angle significantly influence the aerodynamic performance of the inlet duct. Finally, the paper provides recommended ranges for design parameters, offering a foundation for future guidelines in designing high-performance radial inlet ducts.