[Objective] Rill hydraulics is an important dynamic factor affecting slope soil erosion. Rill morphology affects flow dynamics characteristics at all erosion stages, and further on development and morphology evolution of rill erosion.[Method] Scouring experiments were conducted on flumes packed with saturated purple soil for determination of runoff velocity in non-eroded and eroding rill, relative to flow discharge (2, 4, and 8 L·min^-1) and slope gradient (5°, 10°, 15°, and 20°). The electrolyte pulse boundary model method was used to measure rill flow velocity (V), On such a basis, calculation was done of the hydraulic parameters of rills of non-eroded and eroding (including mean flow depth (h), Reynolds number (Re), Froude number (Fr), Darcy-Weisbach friction coefficient (f) and Manning roughness coefficient (n)), and then comparison was made of the hydraulic characteristics between the rills of the two states.[Result] Results show that the flow velocity in the rills of non-eroded state (V_N) ranged from 0.170 to 0.778 m·s^-1, and that in the rills of eroding state (V_E) did from 0.156 to 0.619 m·s^-1. V_N varied with flow discharge to the power of 0.524 and with slope gradient to the power of 0.601, while V_E varied with flow discharge to the power of 0.474 and with slope gradient to the power of 0.530, and their determination coefficient was 0.910 and 0.905, respectively. The mean flow depth of the rills of non-eroded state (h_N) ranged from 0.078 to 0.413 cm, and that of the rills of eroding state (h_E) did from 0.107 to 0.450 cm. h_N varied with flow discharge to the power of 0.502 and with slope gradient to the power of -0.505, while h_E varied with flow discharge to the power of 0.558 and with slope gradient to the power of -0.415, and their determination coefficients were both 0.942. When the flow discharge was 2 L·min^-1, Re was < 500, and when the flow discharge was 4 or 8 L·min^-1, Re varied between 500 and 2 000. Fr was always >1 regardles of working conditions, and ranged from 1.237 to 6.026 in the rills of non-eroded (Fr_N), and from 1.079 to 4.274 in the rills of eroding state (Fr_E). f ranged from 0.079 to 0.856, n did from 0.011 to 0.035, f_N (in the rills of non-eroded state) did from 0.079 to 0.592, and f_E (in the rills of eroding state) did from 0.156 to 0.856.[Conclusion] The parameter h is positively related to flow discharge in the power function, but negatively to the slope gradient. The influences of flow discharge and slope gradient on V decrease in all the rills regardless of state, and that of the former on h increases, while that of the latter on h decreases. The flow regime is dominated by transition flow and rarely by laminar flow, all belonging to rapids. Re does not differ much between rills different in state; Fr_Eremaints almost a constant while approaching 1; f varies as affected by the interaction between V and h, in which V is the dominant one in the rill of non-eroded state, while h is in the other rills. During the development of the rills from the non-eroded state to the eroding state, V decreases, while h increases, so that Re increases, Fr decreases, and both f and n increase, and the erosion of the rills of eroding state caused by rill flow is lower than that of the rills of non-eroded state in intensity. This is because variation of the rills in morphology promotes energy dissipation, which in turn retards rill flow, reduces flow velocity, but increases the mean flow depth, thus eventually leading to changes in flow regime and resistance. All the findings in this study may hopefully help determine hydraulic characteristics of saturated purple soils with rills different in state and be of certain reference value to the establishment of a physical model for researches on soil erosion.