Soil erodibility parameter and critical shear stress are important indicators to evaluate the degree of soil erosion sensitivity and the resistance to the shear deformation of water flow. To date, the change of soil erodibility parameter and critical shear stress is unclear in the rill erosion process of saturated loess soil slope. In this study, the maximum rill detachment rates of saturated loess soil slope were determined by a series of soil scouring experiments in the laboratory under different slopes (5°, 10°, 15°, and 20°) and flow rates (2, 4, and 8 L/min). Then, the numerical, modified numerical, and analytical approaches were utilized to calculate the soil erodibility parameters and critical shear stresses. The results showed that the maximum rill detachment rates calculated by the three methods increased with the increase of slope and flow rate over saturated loess soil slope, and the maximum rill detachment rates calculated by the modified numerical method were similar with those calculated by the analytical method. The soil erodibility parameters were 0.470, 0.278, and 0.256 s/m, respectively, and the critical shear stresses were 1.502, 1.306, and 1.367 N/m2, respectively. The modified numerical method improved the calculation accuracy, thus the modified numerical calculation was close to the theoretical value calculated by the analytical method. The soil erodibility parameters of saturated loess soil slope decreased by 16.83%, and the critical shear stresses decreased by 66.97%, compared with those of non-saturated loess soil slope in the same study area. Soil saturation had no significant effect on the soil erodibility parameters, while greatly reducing soil critical shear stress, and then leading to serious soil erosion on loess soil slope. Besides, the critical shear stresses of the saturated loess soil slope were 6.38% larger than those of the saturated purple soil slope, and the soil erodibility parameters of the saturated loess soil slope were 2.26 times those of the saturated purple soil slope. These results indicated that the soil saturation had similar effects on the critical shear stress of the two soils, while the saturated loess soil was more sensitivity on soil erosion than the saturated purple soil. These findings can provide some references to optimize the rill erosion model parameters in different soil slopes under the condition of saturations.