Abstract: Extensive researches were conducted on the instability mechanisms of surrounding rock in homogeneous strata, however, the effect of soft and hard stratum combinations on the stability remains unclear, to remedy this deficiency, based on nonlinear Mohr-Coulomb failure criterion and upper bound theorem, the collapse mechanism in deep buried tunnel is established with consideration of composite strata, according to equivalent conversion principle between Hoek-Brown and nonlinear Mohr-Coulomb failure criterion, the upper bound solutions are compared with those of existing researches, and the model is proved to be efficient. Three cases including strong rock in upper layer and weak layer in lower layer and vice versa and uniform layer are designed, changes rules of collapsing mechanism are studied, then, reliability function is derived according to the difference between support force and falling block weight, response surface function is established to replace real performance function due to its complication, and reliability index can be calculated by first-order reliability method, subsequently, the evolution rules of failure probability is researched, eventually, according to the principle of reliability-based design, response mechanism of minimum designed support pressure to different parameters is investigated. The results reveal that spatial combination relationship between soft and hard strata directly determines the evolution trend of the instability range of surrounding rock. Different parameters possess varying effects on collapse mechanism and failure probability. In all cases of strong and weak layer combination, strong rock in upper layer and weak rock in lower layer tends to be more unstable. Eventually, the minimum designed support pressure presents a positive correlation with tunnel radius, while a negative relationship with tunnel buried depth in lower layer. The results can provide a theoretical basis for support of surrounding rock in composite strata.