The impacts of O-2 and H2O on the combustion characteristics of pulverized coal in O-2/CO2/H2O atmosphere were studied. The gasification reaction ratio was calculated from the components of flue gas. The competition exists between C-CO2 and C-H2O reactions under rich CO2 atmosphere. At various H2O concentrations, the differences were found in generation amounts of CO and H-2 in flue gas. At O-2 concentrations <10%, C-CO2 reaction decreased while C-H2O reaction increased with increasing H2O concentration; while at O-2 concentration >30%, H2O showed no obvious specific patterns of effects. The gasification ratio was reduced as coal ranks increase.