Abstract: Biological soil crusts (biocrusts) coexist with the herbaceous vegetation in the arid and semi-arid regions. Unique patterns of distribution has formed to collectively influence the soil and water erosion. These ecosystems can be characterized by the unstable soil structures and sparse vegetation cover. The stabilizing functions of biocrusts and vegetation can be expected to mitigate the sediment loss. The hydrological dynamics can also be used to regulate the soil infiltration. However, it is still remained unknow on the role of biocrust distribution patterns in soil erosion, particularly in grassland ecosystems. Therefore, it is a high demand to accurately assess the soil erosion risks for the effective management strategies in these regions. In this study, the simulated rainfall experiments were conducted under consistent vegetation coverage. A systematic investigation was implemented to explore the patterns of biocrust distribution on the soil and water erosion processes under different coverage levels. The underlying dynamic mechanisms of soil and water loss were elucidated under the distribution pattern of biocrusts in grassland. The results showed that: (1) The pattern indices of distribution were significant differences in biocrust with the different coverages. Specifically, the edge density (ED) and Patch cohesion index (COHESION) increased by 1.70 and 1.03 times, respectively, as the biocrust coverage increased from 20%-35% to 35%-50%. Conversely, the splitting index (SPLIT), which represented the degree of patch fragmentation, decreased by 95.4%, indicating the decreasing patch fragmentation. The biocrusts were formed more cohesive and less fragmented patches, as the coverage increased. Some implications were gained for the overall stability and hydrological function of the landscape. (2) The distribution of biocrust was depended significantly on the soil erosion. Importantly, there was the outstanding threshold impact of the biocrust distribution pattern on the runoff and sediment yield. The impact of biocrust distribution on the runoff and sediment yield was significantly reduced, when the SPLIT reached 35.89, corresponding to approximately 44.7% of the biocrust cover. Furthermore, the combined effects of biocrust and vegetation cover were dominated the erosion above this threshold. Further changes in the distribution of biocrust patches shared the less influence on the soil and water erosion. This threshold suggested that the spatial distribution was less influence on the runoff and sediment yields, when the biocrust cover reached a critical point. (3) When the biocrust SPLIT was below 35.89, the distribution patterns of biocrusts were significantly altered the water erosion under dynamics mechanism. The fragmentation was the primary influencing factor on the erosion among these patterns. The velocity of runoff and erosion energy (Reynolds number and runoff power) increased, as the SPLIT of biocrust patches increased (the patches were more fragmented), leading to the higher runoff and soil erosion rates. Therefore, the patch fragmentation was minimized to control the soil and water erosion, when the biocrust cover was below the threshold. The biocrusts and their distribution patterns were highlighted during soil and water conservation in grassland ecosystems. There was the outstanding influence of biocrust distribution patterns on the erosion dynamics, particularly in relation to patch fragmentation and coverage thresholds, in order to better predict and mitigate the water erosion risks. This finding can provide the important insights and scientific basis into the ecological function of biocrusts in the semi-arid regions, particularly for the targeted strategies of soil conservation.