This paper presents an experimental study on bubble formation in the process of sintering of Zhundong coal and corn stalk (CS) ash blends in a horizontal chamber furnace. The effect of the blend ratio of biomass ash was investigated. Bubble parameters, such as number, area, and porosity, were measured on the basis of a metalloscope equipped with a chargecoupled device camera and digital image processing technique. After sintering experiments, ash and condensed matter in the bubbles were sampled and analyzed by X-ray diffraction. In addition, a chemical equilibrium calculation was conducted to reveal the influence of biomass ash on the formation of bubbles. The experimental results show that a 50% CS blend has the greatest melting degree and the formation time of bubbles is earlier than other cases, while a low ratio of CS ash has limited influence on bubble formation and mineral composition. The formation mechanism of bubbles is proposed on the basis of the results. The condensed matter in the bubbles mainly contains NaCl, CaSO4, and KCl. The results indicate that chlorine promotes the transformation of alkali metals to gaseous phase. The chemical equilibrium calculation verified the experimental results.