Abstract: To clarify the triaxial mechanical properties and evolution mechanism of fully cemented unclassified tailings backfill (CUTB) under the action of seepage pressure, triaxial mechanical tests and scanning electron microscope tests were carried out on the CUTB under different seepage pressures, and the influence rule of seepage pressure on the triaxial mechanical properties and meso-structural characteristics of CUTB was analyzed, respectively. Based on damage theory and effective stress principle, the damage modulus constitutive model of the CUTB under seepage pressure was established. The results show that the peak stress and elastic modulus of CUTB linearly decrease with the increase of seepage pressure. However, with the increase of cement-sand ratio, the weakening effect of seepage pressure on peak stress gradually weakens, and the weakening effect on elastic modulus gradually increases. Seepage pressure will degrade the internal structure of CUTB and accelerate the generation and development of micro-pores and micro-cracks. The fractal dimension of CUTB increases with increasing seepage pressure. Based on the experimental data, the damage constitutive model of CUTB under seepage pressure is verified, and the stress-strain curves of the two models are consistent with each other. It shows that the model can well describe the triaxial mechanical behavior of CUTB under seepage pressure. The research results can provide some theoretical guidance for the design of the triaxial mechanical property index of CUTB in water-rich mines.