Abstract: As a herbaceous biomass fuel with high energy content and low ash yield, Arundo donax (AD) exhibits strong potential for synergistic co-firing. To investigate ash evolution and slagging behavior during its co-combustion, experiments were conducted using a laboratory-scale drop-tube furnace with AD blended with Shenmu coal and ordinary mixed coal. Combined with XRF, SEM-EDS, XPS, and ash fusion temperature measurements, the composition evolution, structural characteristics, and potassium occurrence states of the ash were systematically analyzed under different blending ratios and temperatures. Results indicate that the addition of AD significantly increases the proportion of lowmelting components such as K, Cl, and S in the ash, thereby enhancing slagging tendency, with a more pronounced effect in the ordinary mixed coal system. Elevated temperature promotes the transformation of K species from K₂SO₄ to more stable structures like KAlSiO₄, and the ash tends to be densification. Shenmu coal ash, dominated by a SiAl skeleton, exhibits stronger structural stability and better resistance to slagging.