[Objective] To investigate the rill erosion characteristics on slopes under different slope gradients and rainfall-runoff conditions. [Methods] Continuous simulated rainfall and upper slope inflow tests were conducted to study the erosion processes on slopes with two slope gradients (5° and 10°) and across three rainfall-runoff stages (rainfall, inflow, and rainfall + inflow). [Results] The amount of slope runoff and erosion increased with slope gradient. Under rainfall conditions at a 5° slope, the runoff and erosion amounts were 1.06 and 2.23 times than those under inflow conditions, respectively, while at a 10° slope, the runoff and erosion amounts were 1.26 and 3.34 times than those under inflow conditions. The runoff rate initially increased over time and then tended to stabilize. The initial runoff time occurred earlier with increasing rainfall frequency and slope gradient, with both the stable runoff rate and sediment yield rate increasing correspondingly. Under the same slope gradient, the contribution rates of slope runoff and erosion under continuous rainfall conditions decreased with each additional rainfall event. Under continuous inflow conditions, the contribution rate of slope runoff was highest during the first event, reaching 56.40%, while the contribution rate of slope erosion was highest during the third event at 87.50%. Correlation analysis revealed that slope runoff and erosion were highly significantly correlated with the rainfall inflow treatments and the interaction of slope gradient with rainfall inflow treatments (p < 0.001). Variance analysis indicated that the effect of rainfall inflow treatments on slope runoff and erosion was highly significant (p < 0.001), while the effect of slope gradient was moderately significant (p < 0.01). Rill erosion accounted for 37.69%–85.45% and 53.99%–90.65% of the total slope erosion at the two slope gradients, respectively, with rill erosion increasing as gradient increased. [Conclusion] The findings contribute to a deeper understanding of the mechanisms of rill erosion and hold significant implications for soil and water conservation efforts in the black soil region of Northeast China.