<p>The determination of the critical support pressure of the excavation face is one of the significant problems in shield tunnel engineering. In actual engineering, shield tunnels often pass through upper-clay and lower-sand composite water-bearing strata, but there are few researches concerning the excavation face stability of shield tunnel passing through the composite strata under the action of seepage. Based on the numerical simulation and the limit equilibrium theory, the <i>logarithmic spiral and oblique quadrangular prism</i> failure model of shield tunnel excavation face in the upper-clay and lower-sand composite water-bearing strata is proposed depending on response characteristic caused by strata property difference, and the calculation formula of the critical support pressure is derived. Then the influences of water level height, clay-sand thickness ratio, internal friction angle of clay and sand on the stability of shield tunnel face and critical support pressure are analyzed based on the model feature. The results show that the <i>logarithmic spiral and oblique quadrangular prism</i> instability failure model is reasonable and effective. When the water level decreases, the instability failure zone and the critical support pressure decreases, and the logarithmic spiral curve will approximately degenerate to the linear instability failure boundary without water. With the increase of clay-sand thickness ratio, the critical support pressure increases and the pore pressure vacuum zone in front of excavation face decreases. The greater the internal friction angle of clay or sand is, the more stable the excavation face is, and the smaller the critical support pressure is required. Research results can provide a theoretical basis for determining the critical support pressure of the shield tunnel excavation face passing through the upper-clay and lower-sand composite water-bearing strata.</p>