For the randomness and disordered issues of the contact status of earth-rock dam geomembrane and granular cushion, combined with the relationship between the mechanical strength of geomembrane and ability to resist bursting and piercing, a calculation method of probability of bursting and piercing in local geomembrane under simplified conditions is established. Also, this paper analyzed the relation between different granular size and pressures of granular cushion and bursting and piercing probability of local geomembrane. Considering a certain earth-rock dam with impermeable membrane as an example, this paper provided probability distribution of the safety of impermeable membrane structure under overall geomembrane force-deformation condition. The results show that: the greater the pressure, the larger the particle size, the higher the possibility of bursting and piercing of geomembrane, and due to the uncertain imbalance force direction of the local geomembrane, the mentioned relationship presents two functions.