The porous structure of loess can not be eliminated fundamentally by banded and mesh reinforcement materials. Fiber as homogeneous material with holistic structure and isotropy. Fiber-reinforced loess is widely used to achieve three-dimensional reinforcement technology, especially polypropylene fiber (PP fibers) reinforced materials. In this paper, polypropylene mesh fiber and rod fiber are selected as the improved reinforcement materials for collapsible loess from the Chinese Loess Plateau. The effects of the two fibers on the mechanics characteristics of the stiffened loess were investigated by triaxial tests. Fiber length and fiber incorporation amount were selected as control variables to study the optimal fiber length and fiber incorporation amount to effectively improve the strength of collapsible loess. Samples before and after triaxial tests were selected for further scanning electron microscope (SEM) and computed tomography (CT) scanning tests. The interaction between loess and different polypropylene fibers was investigated from the microscopic point of view, that is, the microstructure evolution characteristics of soil-geosynthetics interaction. The strengthening mechanism of PP fibers on collapsible loess strength is clarified, and the optimal scheme of fiber-reinforced soil is determined. The microstructure evolution characteristics showed that the PP fibers can effectively reduce the porosity of loess.