Two kinds of tri-high coals were selected to determine the influences of ash-existing environments and coal structures on CO2 gasification characteristics. The TGA results showed that the gasification of ash-free coal (AFC) chars was more efficient than that of corresponding raw coal (RC) chars. To uncover the reasons, the structures of RCs and AFCs, and their char samples prepared at elevated temperatures were investigated with SEM, BET, XRD, Raman and FTIR. The BET, SEM and XRD results showed that the Ash/mineral matter is associated with coal, carbon forms the main structural framework and mineral matters are found embedded in the coal structure in the low-rank tri-high coal. The Raman and FTIR results show that the ash can hinder volatile matters from exposing to the coal particles. Those results indicate that the surface of AFC chars has more free active carbon sites than raw coal chars, which are favorable for mass transfer between C and CO2, thereby improving reactivity of the AFC chars. However, the gasification reactivity was dominated by pore structure at elevated gasification temperatures, even though the microcrystalline structure, functional group structure, and increase in the disorder carbon were improved by acid pickling.