The application of biochar can modify soil physicochemical properties, thereby influencing soil detachment capacity (D-c), rill erodibility (K-r) and critical shear stress (tau(c)). However, the effects of biochar particle size on D-c, K-r and tau(c) remain unexplored. This research investigated how apple branch-derived biochar with different biochar particle sizes and rates affects D-c, K-r, tau(c), and quantified their relationships with soil physicochemical properties through field experiment. Undisturbed soil samples were collected from field plots treated with biochar at 0 %, 1 %, 2.5 %, and 4 %, and particle sizes of 2-1, 1-0.5, and <0.5 mm for 3 months, using steel rings to a depth of 20 cm. The D-c, K-r, and tau(c) of these samples were evaluated using a flume experiment, where soil samples were subjected to two flow discharge rates (0.25 and 0.65 L s(-1)) and three slope gradients (17.63 %, 26.79 %, and 40.40 %). The results revealed that biochar application significantly reduced D-c and K-r, with the most pronounced reductions observed at the 4 % biochar rate (65 % and 174 %, respectively). Larger biochar particles (2-1 mm) were more effective in reducing D-c (64 %) and K-r (61 %) compared to smaller sizes. The total porosity (TP), cohesion (COH), mean weight diameter of soil aggregates (MWD) and soil organic carbon (SOC) were identified as critical factors influencing D-c and K-r. Power function equations effectively estimated D-c and K-r based on TP, COH, MWD and SOC under different biochar application rates. Similarly, the D-c and K-r were well predicted using power function equations incorporating SOC, soil crust hardness (SH) and >0.25 mm water-stable soil aggregate (SWA) under varying biochar particle sizes. These findings demonstrate that higher biochar application rates and larger particle sizes significantly enhance soil erosion resistance in loess soils, offering a promising strategy to mitigate rill erosion in degraded or degrading sloping farmlands on the Loess Plateau.