Abstract: The exceptionally large foundation cushion for the high arch dam of Jinping 1 hydropower project is 155m in height. The multiple lattice method was used to search for any potential slide plane and to calculate safety factors against sliding of No.1―No.12 sliding blocks, which were concreted in stages and under the normal storage level respectively. The calculation results indicate that the safety factor against sliding of the foundation cushion varies from 1.71 to 2.33 when it is concreted in stages while that under the normal storage level increase to 1.91―2.65 thanks to thrust action of the spandrels, satisfying the stability requirement.