Vegetation litter is important for regulating the hydrological cycle in forested ecosystems. However, whether there is a threshold for litter density that significantly impacts the hydrological process on a hillslope remains unclear. In this study, two typical plantation species on the Loess Plateau of China, namely, Pinus tabulaeformis (coniferous) and Robinia pseudoacacia (broadleaf) plantations, were selected. A total of 22 runoff plots (projected area 2.25 m2) were established, including six treatments of various litter densities (0-1800 g/m2) with two replicates each. The complete processes of rainfall-interception-infiltration-runoff for runoff-generating rainfall events (10-70 mm) in the wet season of 2023-2024 were observed to quantify the threshold of the impact of litter density on hydrological processes and elucidate the underlying mechanism. The results revealed that surface runoff was initiated when rainfall exceeded approximately 10 mm on both types of plantation. Under forest conditions, there were thresholds for the impact of litter density on runoff, while its impact on infiltration and consumption was relatively small. When the litter density exceeded 300 g/m2, the runoff remained almost stable for R. pseudoacacia, whereas for P. tabulaeformis, the threshold of litter on surface runoff was 1200 g/m2. Runoff from the P. tabulaeformis plantations was significantly greater than that from the R. pseudoacacia plantations, averaging 3.75 times. Soil moisture recharge and water consumption were greater in R. pseudoacacia. These are attributed primarily to variations in understory vegetation and litter decomposition rates between the two plantation types. These results support the presence of thresholds in the effect of litter density on runoff, differing between coniferous and broadleaved plantations. P. tabulaeformis plantations, rather than R. pseudo acacia plantations, may increase the runoff in rivers and decrease the loss of rainfall water.