As one of the crucial means of CO 2 emission reduction in coal-fired power plants, oxy-fuel combustion has attracted widespread attention for years. Since oxy-fuel combustion and air combustion vary considerably in the gaseous environment, the gasification reaction has proved to be non-negligible. Exploring the promotion and competition mechanism between oxidation and gasification reactions will help regulate the combustion characteristics of char through pressure and atmosphere. Thence, it is critical to explore the influence of the gasification reaction on oxy-fuel combustion and decouple the contribution ratio of the gasification reaction. The experiments under different conditions are performed on a laboratory-scale pressurized fixed bed in the research. The C atom in CO 2 gas is labeled with 5.95% 13 C; thus, the contribution ratio of the gasification and oxidation reactions can be decoupled by tracking the 13 C atom abundance in CO 2 and CO. The results are apparent: the pressure, temperature, and oxygen concentration increase, the combustion process is promoted, the combustion rate increases significantly, and the burnout time (t 80 ) decreases. In addition, the problem of the low combustion rate under oxy-fuel combustion can be solved by increasing the O 2 concentration. The decoupling results indicate that the contribution ratio of the gasification reaction increases with increasing pressure and temperature. As the CO 2 partial pressure increases, the gasification reaction is promoted. With the increase of temperature, the gasification reaction growth rate is faster than the oxidation reaction. In contrast, the contribution rate shows a decreasing trend due to the increased O 2 concentration promoting the oxidation reaction.