In order to compare the hydraulic characteristics of sheet flow and rill flow on saturated purple soil slope and clarify their critical hydraulic characteristics, indoor artificial simulated rainfall experiments were conducted under the combined conditions of five slope gradients (SGs: 2°, 5°, 10°, 15°, 20°) and three rainfall intensities (RIs: 30, 60, 90 mm/h). On the basis of measuring the velocity of sheet flow, rill flow and critical velocity of shee flow transform to rill flow, the corresponding hydraulic parameters were calculated, including water depth, Reynolds number, Froude number and Darcy resistance coefficient. The results showed that: In different erosion stages, the velocity of sheet flow was between 0.064 and 0.151 m/s and rill flow was between 0.175 and 0.350 m/s. The influence degree of SGs and RIs on velocity and water depth of sheet and rill flow was different. The sheet flow was all laminar and was tranquil when the slope gradient was < 10°. The sheet flow was mostly in the state of tranquil laminar flow. The rill flow was transitional under the conditions of moderate and heavy rain and was rapid when the slope gradient was > 5°. The rill flow was mostly in the state of rapid transitional flow. The Darcy resistance coefficient of sheet flow was negatively correlated with SGs and RIs, while that of rill flow was positively. The critical velocity of sheet flow transform to rill flow was between 0.100 and 0.165 m/s. The velocity would change the slope shape, resulting in the difference of the critical flow pattern and flow resistance with SGs and RIs. This study is of great significance to understand the hydraulic characteristics of sheet flow and rill flow on saturated soil slope, and to further clarify the critical hydraulic conditions for the sheet flow to rill flow transition.