Abstract: The resource utilization of industrial waste slag is a hot issue in the field of material engineering. Glass fiber was used to enhance the mechanical properties of cement based filling mortar. Mechanical tests were carried out to explore the influence of fiber content and length on strength and the mechanism of glass fiber was analyzed through microscopic tests. The results show that the compressive and flextural strength of cement-based filling mortar first increases and then decreases with the increasing fiber content due to its reinforcement. The peak value of strength increase is 1.2%, and the middle length of glass fiber is the most beneficial to the strength enhancement. When the glass fiber content increases from 0 to 1.2%, the shear strength of the specimen increases by 75.7%, and the brittleness decreases when shear failure occurs, and the structural integrity is improved. The "bridging effect" of glass fiber enhances the adhesion force and friction force of hydration gel, inhibits the propagation of failure crack, and promotes the improvement of mechanical properties. This study can provide an important reference for the safe construction of backfill mining engineering and the resource utilization of glass waste residue.