Natural fibers are environment-friendly and efficient for soil reinforcement. Many studies have reported the influences of fiber percentage on the shear strength of fiber reinforced soil. However, different natural fibers perform differently in the soil reinforcement because of their different physical and chemical properties. In this study, the physical and chemical properties of natural fibers (i.e., natural moisture content, specific gravity, breaking tensile strength, breaking strain, cellulose, hemicellulose, lignin, and ash) were examined for their influences in the soil reinforcement. Experimental data of unconfined compressive strength (UCS) for three types of natural fibers (coir, jute, and water hyacinth) were collected from the literature. A total of 11 factors including soil moisture content, soil density, and fiber percentage were evaluated by using the Bayesian nonparametric general regression (BNGR) method. The robustness of the BNGR algorithm was validated using k-fold cross-validation. A parametric study was carried out to unveil the effects of these input variables. The results indicated that a higher fiber percentage or larger soil density induces higher reinforcement effect. Besides, the cellulose in a natural fiber is found to have a significant effect on the UCS, followed by the ash, whereas the other chemical components, i.e., hemicellulose and lignin, have less association with soil reinforcement. With respect to the physical properties, the breaking strain is found to be highly correlated with the UCS, whereas the effects of fiber natural moisture content, specific gravity, and breaking tensile strength are less significant parameters.