Abstract: ［Objective］ To effectively prevent lining damage and water leakage in tunnels, research is carried out on minimizing lining structure bending moment for tunnel cross-section in upper-soft lower-hard composite strata. ［Method］ Soft soil-hard soil and soil-rock two types of loading modes for composite strata tunnel lining structures are proposed based on composite strata tunnel surrounding rock pressure characteristics and reasonable assumptions. Following the ideal optimization approach for a zero bending moment composite strata tunnel, a semi-structural mechanics model of tunnel cross-section is established. The rational axis equation of the zero bending moment tunnel cross-section and the analytical expressions of diameters at both composite strata boundary lines and the horizontal tunnel cross-section mid-line are calculated. The process of minimizing bending moment in composite strata tunnel cross-section is outlined briefly. ［Result & Conclusion］ The rational axis of the zero bending moment cross-section in soft soil-hard soil tunnel is pear-shaped, while in soil-rock tunnel, it is horseshoe-shaped. In practical engineering, based on the rational axis equation of tunnel cross-section, key parameters for tunnel cross-section design can be determined through comprehensive evaluation and weighted averaging method. This approach enables the design of new tunnel cross-sections in upper-soft and lower-hard strata, aiming to minimize lateral deformation in composite strata tunnels.