In response to the national energy strategy and to meet the demand for environmental protection, the NH3-coal co-firing technology proposed to provide a new technical route for power plant researchers. In this paper takes the 650MW ultra-supercritical tower boiler produced as an object, numerical simulation calculations were carried out on the boiler using CFD hydrodynamic simulation software to study the combustion characteristics and stable combustion of the boiler under different ammonia doping ratios. The simulation results show that, compared with the pure coal condition, because of the low theoretical flame temperature of NH3, the low-temperature jet is prolonged, the ignition is delayed, and the burner temperature nozzle area is reduced to a certain extent; in the case of 60% ammonia doping in a large proportion of the operating conditions, the average temperature along the height of the chamber is always lower than that of other conditions, which demonstrates that it is not the higher the proportion of ammonia doping that is the better; the distribution of CO molar volume fraction and O2 volume fraction in each ammonia doping ratio shows an opposite trend, and the addition of NH3 has little effect on the CO2 emission concentration because NH3 is a fuel without carbon .In addition, in the study of the effect on the combustion stability of the boiler under each ammonia doping ratio, it was found that a high ammonia doping ratio was unfavourable to the combustion of the boiler.