Abstract: Currently, the fundamental combustion characteristics of the Allam cycle under high-pressure CO2-rich environments remain unclear, and existing detailed reaction mechanisms are relatively complex with low computational efficiency, which constrains the design and optimization of related combustors. This work focuses on the CH4/O2/CO2 system to study combustion characteristics and mechanism reduction. Using sensitivity analysis and the direct relation graph method, the GRI3.0 mechanism suitable for atmospheric pressure and the Aramco3.0 mechanism suitable for high pressure were respectively reduced. Validation was performed over a pressure range of 0.1–10 MPa using laminar flame speed and ignition delay time as metrics, ensuring errors within 5%. After reduction, the species and reaction counts in the GRI3.0 mechanism were decreased by about 40%, while those in the Aramco3.0 mechanism were reduced by about 55%. Simulations based on the reduced mechanisms show that, due to the inclusion of key intermediates such as CH3O2 and CH3O2H, the Aramco3.0 mechanism differs significantly from the GRI3.0 mechanism in reaction pathways and conversion rates. Analysis of combustion characteristics reveals: laminar flame speed is positively correlated with initial temperature, and its sensitivity to pressure increases with rising temperature; its variation with equivalence ratio first increases and then decreases at low pressure, while it increases monotonically at high pressure. Ignition delay time is negatively correlated with initial temperature and decreases more slowly as pressure rises. This study provides an efficient and reliable mechanistic tool for combustion simulation and optimization of the Allam cycle, and also lays a theoretical foundation for research on supercritical CO2 power cycles.