Abstract: The ultra-deep overburden foundation is a great challenge for the construction of water conservancy and hydropower projects. The dynamic mechanical properties of overburden soil are the basis for discriminating liquefaction of foundation soil and analyzing dam stability. The sand layer in the ultra-deep overburden foundation of a proposed large-scale earth rock dam project is taken as the research object. A series of undrained cyclic triaxial tests are conducted on the undisturbed soil samples (directly extracted from an ultra-deep in-situ test well) and remolded soil samples under high confining pressure conditions. The laboratory results demonstrate that the dynamic strength of the undisturbed sand is higher than that of the remolded sand, and the difference between them gradually decreases with the increase of the confining pressure. The evolution laws of pore pressure and axial strain of the undisturbed and remolded soil samples are similar. The consolidation stress ratio has obvious influences on both the dynamic strength index and the failure mode of the soil samples. With the increase of the consolidation stress ratio, the dynamic strength increases, and the failure mode of the soil samples gradually changes from liquefaction failure to plastic strain accumulation one.