To study the stability of the tunnel face for dual-mode shield machine excavated in a composite strata, a two-dimensional failure mechanism of the tunnel face was constructed by using spatial discretization technique. The objective function of the soil chamber pressure in the limit state was derived by using this failure mechanism and the upper limit theorem of limit analysis. The upper bound solution of the soil chamber pressure to maintain the stability of tunnel face during the shield machine pass through the interface of the soil/rock was obtained from optimization. Moreover, the influence of different parameters on the soil chamber pressure and the failure mode of the tunnel face were discussed. On this basis, based on the probabilistic theory, the reliability model of the soil chamber pressure was established, and the minimum soil chamber pressure which was used to maintain the stability of tunnel face in composite stratum with a tolerable reliability was given. Parametric analysis shows that the soil chamber pressure decreases as the distance between the shield machine and interface of the soil/rock shortens, and failure region in front of the tunnel face increases with the increase of the interface inclination angle.