With the acceleration of urbanization in the loess region of western China, construction land with fill and excavation is abundant and requires the corresponding foundation treatment technology to solve the problem of large deformation in the filling area. In this paper, fly ash and geogrid were used as modified fillers to improve loess. Through the direct shear, triaxial, collapsibility, and scanning electron microscopy tests, the strength change rule, macroscopic damage characteristics, and microstructure characteristics of reinforced and unreinforced loess specimens with different mixing ratios of fly ash were analyzed. The results show that the optimum water content and maximum dry density of the fly ash-modified loess reinforced by geogrid increases and decreases, respectively. When the mixing ratio of fly ash is 20%, the reinforced improvement effect is the best: The peak strength, residual strength, and equivalent internal friction angle of loess are greatly improved, and the lateral deformation and collapsibility are greatly reduced. Scanning electron microscopy test results show that the improvement of loess by fly ash is mainly reflected in the direct filling of loess pores by fly ash particles and the interweaving of chemical crystals to form a network filling; the porosity and pore size of loess decrease after the improvement. The study results provide a reference for improving and reinforcing fill slopes and fill roadbeds in loess areas.