Abstract: The screw piles were used in some civil engineering projects.But some problems of designing including failure model of soils,the design of shape of screw piles,the computation of ultimate load for single screw pile or screw pile group,the reasonable ratio of diameter of lamina to pitch of screws or the distance of laminas were seldom researched.By the analysis of the failure model of screw piles under the ultimate load,the pivotal question of designing pile shape was the design of pitch of screws,so the failure model of soils was advanced and the relation of the diameter of lamina with the pitch of screws was deduced and defined as the controlled pitch of screws and the minimal pitch of screws.The computational method of ultimate load of single pile and pile group was proposed for the different pitch of screws.Compared these computed values of ultimate load with those obtained with static load test of 6 pile groups,the computated values were close to those from tests,so the failure model,and the computational method of ultimate load of pile and pile group could be used in design to estimate ultimate load of pile or pile group.