Abstract: Biggish creep deformation was found at Guanyinyan composite dam composed of earth dam and concrete dam after construction, which induced cracks in the earth-rockfill dam in a month after initial water impounding. According to the settlement data after construction completion, the creep deformation parameters of the rockfill material and clay soil used in the dam were back-analyzed using a parallel mutation particle swarm optimization. According to the back-analyzed creep deformation parameters, finite element analysis of the dam is carried out, by which the deformation features of the dam in the impounding period and operation period are analyzed and calculated respectively. The results show that the main causes for dam cracks are bigger uneven settlement and creep deformation of the dam.