To investigate the effects of different plants on soil and water conservation in ephemeral gullies of agricultural fields in the black soil region of Northeast China, ephemeral gullies were modelled on bare slopes (LP) and slopes planted with both alfalfa (V₁) and Iris ensata Thunb (V₂). By simulating upper catchment, hydraulic erosion experiments were carried out on the slope surface of ephemeral gullies covered by different plants, and the characteristics of runoff and sediment yield as well as the spatial and temporal variation of hydrodynamic parameters were analyzed. The results showed that: (1) Under the same flow rate, the runoff generation time followed the order of LP < alfalfa < I. ensata Thunb, with significantly prolonged flow production and greater infiltration, sediment-reduction benefits of alfalfa (61%) was significantly better than that of I. ensata Thunb (29%), but the runoff reduction benefits of alfalfa (33%) was slightly weaker than that of I. ensata Thunb (38%). (2) Under the same flow rate, hydraulic friction coefficient (f) of alfalfa (2.3) was higher than that of I. ensata Thunb (0.8), while the average flow rate (u) of alfalfa was slightly lower than that of I. ensata Thunb, the Froude number (Fr) and the mean Reynolds number (Re) followed the order of alfalfa < I. ensata Thunb, and the overall flow was turbulent. (3) Analysis of the fitted functions showed that Reynolds number was positively correlated with flow rate and friction coefficient under the three sets of experiments, but the functional relationship was slightly different. Froude number was negatively correlated with friction coefficient, all in line with the law of logarithmic function change. The results showed that the reduction of runoff and sediment yield was due to the increase of friction and infiltration rate of the vegetation cover, and alfalfa was more effective in reducing runoff and sediment than I. ensata Thunb. With the increasing flow rate, the runoff and sediment reduction benefits of I. ensata Thunb decayed more rapidly than those of alfalfa. The critical flow rate for protection of I. ensata Thunb was 30~40 L/min, and alfalfa could meet the soil and water conservation needs at higher catchment flow.