Abstract: The uniqueness of the sand cyclic unloading envelope is of great importance in understanding the inherent properties of the sand. In this paper, a series of triaxial compress tests and triaxial cyclic tests are conducted to explore the consistency between the stress-strain envelopes of cyclic load with equal stress amplitude, step stress amplitude and random stress amplitude and the corresponding monotonically loaded stress-strain curves based on the digital image measurement system for full surface deformation of sand samples and silica powder. The results show that the stress-strain relationships of cyclic load for different soils with different cyclic stress histories return to the stress-strain curves of monotonic loading. Accordingly, the uniqueness of the stress-strain envelope of sand is proposed, that is, the stress-strain curves of monotonically loaded sand samples under the same density and surrounding pressure conditions are in approximate agreement with the cyclic load stress-strain outer envelopes undergoing different cyclic stress histories. The effects of cyclic stress histories and stress levels on the rate of curve regression are discussed. The application of the uniqueness of the stress-strain envelope in building the axial strain accumulation model is presented in conjunction with the axial strain accumulation behavior under cyclic load.