The high ash content and low calorific value of Shanxi weathered coal limit its utilization in combustion and electricity genera⁃tion. Extraction and utilization of humic acid from weathered coal has been extensively studied, but few studies have been conducted on itsash formation characteristics. To study the ash-slag formation characteristics of the weathered coal, a small fluidized bed reactor was usedas the main experimental device to conduct hot combustion tests on weathered coal under different coal particle size and combustion atmos⁃phere. Balance, vibrating screen, muffle furnace, laser analyzer and X-ray diffraction analyzer were used to analyze the slag-ash ratio,particle size distribution and mineral contents, which revealed the differences in the ash-slag formation characteristics of weathered coal.The results show that with the increase of coal particle size, the particle attrition in the furnace is inhibited and the fly ash ratio decreasesfrom 43.20% to 1.51%, and the production of Quartz, Mullite and Hematite content and the conversion of Kaolinite increase. The increaseof oxygen concentration in flue gas promotes the conversion of Kaolinite and Pyrite into Quartz, Mullite and Hematite: as oxygen concen⁃tration increases from 0 to 5.16%, Quartz content increases from 41.21% to 51.13%, Mullite content increases from 7.24% to 21.42%,and the Hematite content increases from 6.81% to 23.32%. High ash and low calorific value weathered coal can achieve stable combustionin fluidized bed, and when the oxygen concentration of flue gas is between 3%-5%, 2-4 mm weathered coal can maintain high combus⁃tion efficiency and mineral conversion rate, which is conducive to the stable operation of the system and secondary utilization of ash-slag.The result can be used for the fuel-selection of circulating fluidized bed.