Abstract: For the shield-tunneling in sand-gravel composite stratum, if the support force is too small, it will easily lead to active destruction of soils in front of the excavation face and cause surface settlement. A parameter for the sand-gravel composite stratum is defined, that is the ratio of height of silty fine sand in excavation face to the cutter diameter of shield machine. Its effects on the ultimate support force, surface settlement and disturbance range of excavation face instability are analyzed. Based on the model tests and the silo theory, a computational model for the ultimate support force of excavation face of shield tunnels is established. It is introduced into the model, and the relevant formula is derived. The results show that: (1) With the decrease of the support force of the excavation face, the lateral earth pressure-support force curve exhibits the laws of insensitive stage, rapid declining stage, slow declining stage and stable stage. (2) The larger the value of σ is, the less conducive the control of surface settlement is. (3) Compared with that by the traditional model, the failure surface being approximately a broken line by the proposed method is more suitable for the deformation and failure mode of sand-gravel composite stratum. (4) The ultimate support force increases in an approximate linear functional relationship with the increase of σ. When the depth of overburden is large, the effects ofσ on the ultimate support force should be paid more attention to. (5) In the shallow stratum, the effects of overburden depth on the ultimate support force are more obvious, and need to be paid attention to. The research results have important guiding significance to determining the ultimate support force of excavation face in sand-gravel composite stratum.