The effects of different kinds (cotton stalk, rice husk, and sawdust) and proportions (0%, 10%, 20%, and 30% based on weight) of biomass and operating conditions (temperature and excess air coefficient) were evaluated relative to the ash deposition characteristics during the co-firing of Huang Ling (HL) coal with biomass. The experiments were performed in a drop-tube furnace. The chemical compositions and mineral phase characteristics of the collected ash particles were analyzed using scanning electron microscopy with energy dispersive X-ray (SEM-EDX) and X-ray diffraction (XRD), respectively. The results showed that the most severe agglomeration, from co-firing coal with cotton stalk, was due to the higher content of alkali metals, especially K. The amount of K in the ash increased with an increasing proportion of cotton stalk, and ultimately, agglomeration was more serious. When the combustion temperature increased from 1050 °C to 1300 °C, the dystectic solid compounds were transformed into eutectic compounds. The increased excess air coefficient accelerated the sulfur reaction, but did not relieve the heavy sintering. Consequently, limiting the content of biomass in the fuel blends, maintaining a lower combustion temperature, and a suitable level of excess air were determined to be necessary for the co-firing of coal and biomass.