Ammonia-doped natural gas combustion technology has attracted much attention because of its carbon reduction potential and fuel flexibility, but faces the dual challenges of unstable combustion due to uneven mixing in non-premixed combustion mode and the surge of NOx emission. In this study, a new cyclone doped combustor is designed through structural optimization, and the coupling mechanism of ammonia doping ratio on the flow, temperature and emission characteristics of the optimized combustor is systematically revealed. The optimized combustor synergized with the secondary air return improves the problem of slow flame speed of ammonia, and reduces the area of high-temperature zone and the peak concentration of thermal NO. The study on the effect of ammonia doping ratio shows that: with the increase of ammonia doping ratio from 0 to 0.8, the NO emission shows a non-monotonic tri-peak fluctuation, with the peak value of 757 ppmvd at Rm=0.4 and the minimum value of 445.67 ppmvd at Rm=0.6. The NO generation shows the evolution law of "promoting-suppressing-recovery", and this non-monotonicity comes from the dynamic equilibrium of fuel-type NOx and thermal-type NOx. This non-monotonicity results from the dynamic balance between fuel-based NOx and thermal NOx. The optimized combustor in this study effectively solves the problems of uneven mixing and high NOx emission level in single-stage non-premixed natural gas ammonia doped combustor, and provides a key theoretical basis for the multi-objective optimization of clean fuel doped combustor.