Abstract: Partial or completely replacing fossil fuels with ammonia as a fuel is one of the effective ways to reduce CO₂ emissions. In this paper, based on the flat-flame burner, the effect of ammonia mixing ratio on the fly ash characteristics and gas phase composition in the coal ammonia flame is studied by using thermogravimetric analyzer, SLR camera and other detection instruments. The results show that ammonia doping enhances the convective heat transfer and preheating effect of pulverized coal in the coal ammonia jet, which positively correlates the flame temperature with the ammonia mixing ratio. The magnitude and uniformity of coke crushing stress in the coal-ammonia co-combustion are changed by mixing ammonia. With the increase of ammonia mixing ratio (ammonia mixing ratio is greater than 20%), the fly ash particles are narrowed and elongated, the degree of crushing increases, the number of small particles increases, and the carbon residue rate of fly ash decreases. At the same time, due to the higher combustion reaction rate of small particle size, the peak of weight loss rate changes from single peak to double peak, the peak value increases, and the temperature corresponding to the peak shifts to a lower range during the second stage of the thermogravimetric reaction. With the increase of ammonia mixing ratio, the concentrations of O₂, CO and CO₂ in the coal ammonia flame decrease, while the concentration of NO_x increases significantly, and its formation is affected by the reduction system formed by coke/CO/ammonia radicals. In conclusion, ammonia mixing aggravates the crushing of coke, and the reduction of NO_x emission in coal- ammonia co-combustion remains a key focus for future research.