Abstract: For the low strength and poor stability of water-high filling materials improves the property of it. The unsta- ble failure characteristics of water-high filling materials was studied by stress-strain and creep test. The microstructure of water-high filling materials and the improvement mechanism of it were analyzed by using scanning electron micros- copy (SEM) and different thermal analysis-thermal gravimetry (DTA-TG). It is found that the creep curve has decel- erating and stable stage when stress level is low and it increase accelerating stage when stress level reach critical load. The stress-strain curve has initial deformation, elastic, plastic deformation and disruption four stages. The gel time shortened,compressive strength improved,elastic modulus increased,unstable failure critical load raised and deforma- tion decreased of water-high filling materials after improvement. Owing to more ettringite crystals and alumina gel in the water-high filling materials after improvement,the microstructure of hardening body become more densification and bearing capacity improved.