Abstract: The effects of combustion time on the characteristics of mineral distribution and deposition behavior were carried out in a self-designed drop-tube furnace during oat straw combustion. The thermodynamic equilibrium calculations were used to predict the results. Meanwhile, the ashes of different combustion time at 850℃ were analyzed by XRD, SEM/EDS and EPMA. The results show that the initiative formations of oat straw deposition are alkali metal species which can cohere with fuels, then form particles with different sizes. Holes can be formed in these particles surface by internal oxidation reaction. It is found that potassium feldspar is the main species which acts as an important role in cementing action and supporting structure. There are many K and Al species enriched in the initial deposition. In addition, Na can promote the activity of K in some degree. Some K and Na species in the deposition volatilizes into the gas-phase gradually as the combustion time extends. The relevant compounds of Fe, Ca and Mg continue to fill the interspaces and make the structure closed-grain further. Finally, most elements are distributed uniformity in the deposition.