As a carbon-free fuel, ammonia is one of the alternatives to traditional fossil fuels, but its combustion characteristics are poor, and it is usually optimized by blending methane and increasing oxygen content. However, there are few relevant studies under different conditions under high temperature and high pressure conditions. In this study, the laminar burning velocities (LBV) and flame instability of NH3/CH4/O2/N2 mixtures at high initial temperature (T), high initial pressure (p), various oxygen contents (Ω) and methane energy ratios (α) are analyzed using a constant volume combustion chamber (CVCC). According to numerical simulation, how various oxygen contents and methane energy ratios affect the combustion characteristics of NH3/CH4/O2/N2 mixtures and NOx emission is analyzed. The results show that LBV is positively correlated with T、α、and Ω, and negatively correlated with p. Markstein length (Lb) does not change significantly with T, but increases with α and decreases with p and Ω. Both oxygen enrichment and methane blending significantly increase the hydrodynamic instability of the flame and decrease the buoyancy instability of the flame. Oxygen enrichment and methane blending have little effect on the thermal diffusion instability of the flame. Oxygen enrichment and methane blending are effective in increasing the generation of NHi, O, H, OH, etc. radicals and the chemical reaction rate. The main factor of oxygen enrichment and methane blending to increase LBV is thermal effect, but it also leads to the increase of NO, which mainly is thermal NO. The reaction path and sensitivity analysis of NO show that HNO + H <=> NO + H2、N + NO <=> N2 + O play an important role in the production and consumption of NO, respectively. With the change of Ω and α, the concentration of NO production increases and the time of NO production decreases.