Abstract: In order to study the influence of size effect of filling pillars with modified high-water material on the load-bearing capacity 0.2%polypropylene fiber was added to the samples of high-water filling pillar material for modification. By comparing the uniaxial compression test results,failure process and morphology of the modified and unmodified samples,the size effect and failure characteristics were analyzed. Finally,the test results were further discussed by scanning electron microscopy(SEM). The results show that the size effect characteristics of the high-water-filled pillars remain unchanged. With the increase of height-diameter ratio,peak strength and peak strain gradually decrease and the elastic modulus gradually increases. The modified samples’ load-bearing capacity increases,and the peak strength increases by 10.78%,the peak strain increases by 28.62%; the failure types of samples are not affected by height-diameter ratio and the fibers,all of which belong to “X” shear failure of splitting type. Due to the fiber’s crack resistance effect,the integral compressive stability of the modified samples is significantly improved. SEM microscopic morphology comparison shows that the fibers prevent the development of original cracks while delaying the generation of new cracks,so that the material has a more stable ettringite skeleton structure,which indirectly increases the strength and performance of the material and show a higher residual strength.