Physicochemical properties (i.e., morphological variation, functional groups, and carbon structure change) and alkali and alkaline earth metal retention of chars during coal and corn stalks (CS) copyrolysis were investigated in this research. The copyrolysis char samples were obtained at the temperature ranging from 400 to 1000 degrees C at intervals of 100 degrees C under argon atmosphere by a horizontal furnace. The results indicate that the synergistic effects on char yields are slight or even negligible. The presence of CS promotes the formation of plastic mass during copyrolysis because of the interactions between volatiles and chars of coal and CS. As the pyrolysis temperature increases from 400 to 1000 degrees C, the aromatic C-H functional groups first increase and then decrease, and the char structure changes from a small aromatic ring system to a large aromatic ring system containing six or more fused benzene rings. The average number of hydrogen atoms per aromatic ring and the number of the aromatic ring structure with high substitution degree are highest at 500 degrees C. In addition, the C=0 and aliphatic C-H functional groups in the char samples disappear when the pyrolysis temperature ranges from 600 to 700 degrees C. The release of K and Mg is inhibited during copyrolysis, while the release of Ca is promoted.