Experiments were conducted on a laboratory drop tube furnace for the single-firing of ammonia and coal, as well as co-firing, combined with numerical simulations to investigate the NO formation characteristics, intermediate reaction processes and ammonia-nitrogen conversion behavior in ammonia-coal co-firing. The results showed that the NO concentration generated under ammonia-coal co-firing conditions was much higher than that of ammonia and coal combustion alone, and higher than the sum of ammonia and coal combustion alone. With an ammonia co-firing ratio of 45% (by heat value, the same below), the NO emission of ammonia-coal co-firing was 70.17% higher than the sum of ammonia and coal combustion alone. When the co-firing ratio remained the same and the fuel mass doubled, NO emission increased by 79.36%, indicating that co-firing of coal and ammonia led to an increase in NO emissions. The simulation results showed that the concentration of NO in the reactor increased rapidly after ammonia co-firing, and ammonia began to oxidize and generate NO. Ammonia oxidation reaction and ammonia reduction reaction occurred simultaneously, and since the ammonia oxidation rate was always higher than the ammonia reduction rate, the NO concentration increased. After ammonia-coal co-firing, the peak value of the average reaction rate of ammonia-related reactions increased, and the peak value appeared earlier, which promoted the conversion of ammonia-nitrogen to NO.