Abstract: The uniqueness of the stress-strain envelope curves of sand under cyclic unloading is of great importance in understanding the inherent properties of the sand. A series of monotonic triaxial compress tests and cyclic triaxial tests, considering constant amplitude, step stress amplitude as well as random stress amplitude, are conducted on the silica powder and Fujian standard sand to explore the consistency between the stress-strain envelope curves and monotonic curve by employing the full-surface digital image measurement system. The results show that the monotonic stress-strain curves of the soil samples with the same density and confining pressure are approximately consistent with the stress-strain envelope curves that undergo different cyclic stress processes, and the uniqueness of the stress-strain envelope curves of the sand is proposed. The influences of the cyclic stress history and stress level on the regression velocity of the curves and the application of the uniqueness of the stress-strain envelope curves in the establishment of the axial strain accumulation model are discussed.