This study investigates the mechanical-response mechanism of double-layer linings in an underwater large-diameter composite double-layer shield tunnel on the Chongtai Yangtze River Tunnel of the Shanghai-Chongqing-Chengdu High-Speed Railway, China. The analysis employs a load-structure model to study the layer lining responses under different inner-lining construction timings and riverbed scouring actions. Main findings are as follows: (1) Inner-lining construction markedly reduces and increases the bending moment of the lower and upper halves of the segmental lining, respectively. It has minimal effect on the maximum positive bending moment and axial force of the segment and the induced radialcontact stress between the segment and the inner lining is negligible. (2) Earlier construction of the inner lining should be delayed to reduce radial-contact stresses, bending moments, and axial forces after ground stabilization. (3) As the riverbed scouring depth increases, the internal forces and deformations decrease in the segment and increase in the inner lining, accompanied by a continuous reduction of ground resistance at the arch waist. While the arch waist resistance remains greater than zero, the internal forces and deformations of the segment and inner lining vary linearly with scouring depth. Once the resistance reduces to zero, the linear trend is broken by dramatic changes in the mechanical responses of the segment and inner lining. Quasilinear variations, but with a markedly reduced amplitude, persist with further scouring.