Abstract: In arid and semi-arid regions, biological soil crusts (biocrusts) coexist with herbaceous vegetation, forming unique distribution patterns that collectively influence soil and water erosion processes. These ecosystems, characterized by unstable soil structures and sparse vegetation cover, rely heavily on the stabilizing functions of biocrusts and vegetation to mitigate sediment loss, affect soil infiltration, and regulate hydrological dynamics. However, the role of biocrust distribution patterns in soil erosion processes remains poorly understood, particularly in grassland ecosystems. This knowledge gap hinders the accurate assessment of soil erosion risks and the development of effective management strategies in these regions. To address this gap, we conducted simulated rainfall experiments under consistent vegetation coverage to investigate how biocrust distribution patterns affect soil and water erosion processes under different coverage levels and to elucidate the underlying dynamic mechanisms. The results showed that: (1) The distribution pattern indices were significant differences in biocrust with different coverages. Specifically, as the biocrust coverage increased from 20%～35% to 35%～50%, Edge density (ED) and Patch cohesion index (COHESION) increased by 1.70 and 1.03 times, respectively. Conversely, the splitting index (SPLIT), which represents the degree of patch fragmentation, decreased by 95.4%, indicating that patch fragmentation decreased. This result indicated that biocrusts form more cohesive and less fragmented patches as coverage increases, which could have implications for the overall stability and hydrological function of the landscape. (2) Biocrust distribution significantly affected the soil erosion processes. Importantly, the impact of the biocrust distribution pattern on runoff and sediment yield exhibited an obvious threshold effect. When the SPLIT reached 35.89, corresponding to approximately 44.7% of the biocrust cover, the impact of biocrust distribution on runoff and sediment yield was significantly reduced. Above this threshold, the combined effects of biocrust and vegetation cover dominated the erosion processes, and further changes in the distribution of biocrust patches had less influence on soil and water erosion processes. This threshold effect suggested that when biocrust cover reaches a critical point, its spatial distribution becomes less influential in determining runoff and sediment yields. (3) When biocrust SPLIT was below 35.89, distribution patterns of biocrusts significantly altered the water erosion process by affecting erosion dynamics. Among these patterns, fragmentation was the primary factor influencing the erosion process. As the SPLIT of biocrust patches increased (the patches became more fragmented), runoff velocity and erosion energy (Reynolds number and runoff power) increased, leading to higher runoff rates and soil erosion rates. Therefore, in conditions where the biocrust cover was below the threshold, minimizing patch fragmentation could be an effective strategy for controlling soil and water erosion. This study highlights that biocrusts and their distribution patterns play a crucial role in soil and water conservation in grassland ecosystems. Understanding how biocrust distribution patterns influence erosion dynamics, particularly in relation to patch fragmentation and coverage thresholds, allows land managers to better predict and mitigate water erosion risks. This research provides important insights into the ecological function of biocrusts in semi-arid regions and offers a scientific basis for developing targeted soil conservation strategies.