As a typical discontinuous medium, the boulder-cobble mixed strata (BCMS) are often faced with the problems of easy collapse and difficult to effectively support in the construction of large section tunnel, so it is necessary to study the excavation failure modes of tunnel in the BCMS. In this paper, based on a highway tunnel through BCMS in Tibet, the failure modes of the tunnel were analyzed by distinct element PFC^2D program. The results indicate that there were three kinds of excavation failure modes in the boulder-cobble mixed tunnel, that were, small scale local damage, collapse failure of rapid development and failure of cavity rising slowly. The mechanical mechanism of failure was mainly the shear and tensile failure of the contact surface between particles. In addition, to avoid the damage of the tunnel in the BCMS, it was necessary to timely support and reduce the disturbance in the construction.