Fiber content is an important parameter affecting reinforced soil. In existing research, the range of fiber content values is generally small, and the physical and mechanical properties of reinforced soil with higher fiber content are not well understood. In order to study the impact of higher fiber content on rice straw fiber-reinforced soil, this study sets up 10 fiber mass fractions and uses direct shear tests for research, exploring the effect of fiber content on the strength characteristics of reinforced soil. Additionally, scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR) tests are used to analyze the microstructure features of fiber-reinforced soil. The results show that adding fibers can effectively improve the shear strength of the soil. The reinforcement mechanism of fiber-reinforced soil is related to fiber content. A small fiber content has little effect on the porosity of the soil, and the reinforced soil is based on the soil skeleton. When the fiber content is high, the porosity increases significantly, and the main structure of the reinforced soil would be composed of both the soil skeleton and the fiber skeleton.