[Objective] This study aimed to clarify the mechanisms by which biological soil crust（BSCs） influence overland flow resistance, focusing on the effects of mixed biocrust communities on grain resistance（f_g） and form resistance（f_f）. [Methods] A total of 84 undisturbed soil samples with three different mixed BSCs compositions（algae-dominated, equal algae-moss ratio, and moss-dominated） were collected, flume scouring experiments under seven combinations of slope gradient and flow discharge were set up, and the response of overland flow resistance characteristics to BSCs was systematically investigate under different hydrodynamic conditions.[Results] 1) Biocrust coverage reduced the velocity of overland flow by 8.5%-57.3% and increased water depth by 5.4%-70.6%. The flow slowing effect of mixed BSCs was enhanced significantly with the increasing proportion of moss crust, with the flow slowing effect of moss-dominated community（T3） being 1.1 to 2.5 times greater than that of other mixed BSCs. Increased moss proportion reduced Froude number（Fr） and increased Reynolds number（Re） without altering the flow pattern or flow regime. 2) Coverage of mixed communities of biological crust significantly increased the total resistance（Darcy-Weisbach resistance coefficient） f, which decreased as a power function with the increase of Re and Fr（R²≥0.46, p＜0.01）,and showed a significant positive correlation with the biomass of moss crust(M_moss)（ p＜0.05）. 3) BSCs coverage significantly increased the f_f of overland flow, which was the dominant part of the f of overland flow under different slope gradients, flow discharges, and BSCs coverage conditions. The f_g was significantly correlated with sediment load（q_s）（p＜0.05）, and f_f was positively correlated with M_moss（p＜0.05）. 4) The f_g calculated by Savat model was not applicable to overland flow with low q_s under BSCs coverage. Based on the influencing factors of f_g and f_f, a new equation for the f of overland flow under BSCs coverage was established: f=Re^{-1.021 4}×M_moss^0.311×10^3.793. [Conclusion] Under BSCs coverage, the main reason for the increase of the f of overland flow is the increase of the f_f, which is mainly affected by the proportion of moss crust. The study is of great significance for clarifying the mechanism of BSCs affecting overland flow resistance, and provides a theoretical basis for using BSCs to prevent and control soil erosion on the Loess Plateau.