The reliability analysis of a reinforced concrete containment of Qinshan Nuclear Power Plant under seismic excitation is carried out.The system reliability against ultimate failure is evaluated by the stable configuration approach (SGA) and the three dimensional finite element mehtod (FEM) of stress analysis. The failure of the structural system is formulated as the intersection of damages to the stable configuration. The input seismics loading is chosen corresponding to 2SSE earthquake. The ground motion is assumed to be a zero mean stationary Gaussian process with a Kanai Tajimi spectrum. Based on the computed conditional failure probabilities of containment under various peak ground accelerations, the probabilities of ultimate system failure as well as initial yielding per year are given.