Abstract: Four major failure modes for high arch dams, namely destabilization dam lost stability failure, shearing- slipping failure, excess cracking and man-caused destruction, are presented by means of failure tree analysis. Based on the calculation of sheafing-slipping failure probability of high arch dams, the calculation method and procedures are presented by use of the Second Moment Method for calculation of the occurrence probability of the major failure modes of high arch dams, and the relation degree between the major failure modes and the failure probability with multi-failure modes of the high arch dam are calculated.