This study investigates the ash deposition behavior of anthracite during combustion, which significantly affects boiler safety and efficiency due to slagging tendencies. A pilot-scale one-dimensional settling furnace system was employed to conduct combustion experiments under varied operating conditions, including different loads, primary/secondary air ratios, and excess air coefficients. Ash samples were analyzed by fusion tests, SEM, XRD, XRF, and laser sizing. Results show ash composition (C, O, Si, Al) and crystalline phases (quartz, mullite, hematite) remain stable. Increasing the load from 0.2 MW to 0.3 MW raises the ash deformation temperature from 1259 °C to 1312 °C, while the slagging index increases from 1268 to 1319, indicating a significantly enhanced slagging tendency. When the mass ratio of primary air to secondary air is increased from 2/8 to 4/6, the unburned carbon content in ash increases from approximately 37.5% to 40%, and the median particle size enlarges from 22 μm to about 28 μm, resulting in a pronounced promotion of ash deposition. Excess air coefficient had limited impact on fusibility and slagging. Ash exhibited a bimodal size distribution: fine particles form an adhesive layer, while coarse particles deposit by impaction, jointly accelerating slagging. The results demonstrate that boiler load dominates slagging behavior, with air distribution affecting burnout and particle characteristics. This study provides pilot-scale experimental data and mechanistic insights for slagging prediction and combustion optimization of anthracite-fired boilers under wide-load operation.