Ammonia is an efficient hydrogen carrier that is expected to become the next generation of carbon free fuel. However, its use is hindered due to the narrow combustion limit and high NOx emission. Plasma is an efficient and low energy consumption method to enhence combustion, which has been widely used. A self-made ammonia/air premixed swirl combustion platform coupled with dielectric barrier discharge was designed and constructed. The changes of combustion limit, flame shape, O2 and NOx were studied. The limit of lean combustion is 0.75 and the limit of rich combustion is 1.1-1.2 without discharge. When the equivalent ratio is 0.9, the flame propagating speed is the fastest and the flame is the shortest. The flame fills the entire combustion chamber near the low/rich combustion limit. When the equivalent ratio is 1.05-1.10, NO emissions remain at a low level and NO2 is not generated. When the equivalent ratio decreases from 1.05 to 0.75, the volume fraction of NO increases from 97×10-6 (3.5% O2) to 2 785×10-6 (3.5% O2). As the equivalent ratio decreases, a small amount of NO is further oxidized to NO2, and the volume fraction of NO2 reaches 171×10-6 (3.5% O2) at the equivalent ratio of 0.75. The dielectric barrier discharge significantly enhances the combustion reaction and inhibits the negative effect of flame quenching at the wall of combustion chamber. The O2 decreases slightly and the combustion limit extends to 0.65-1.30. Simultaneously, NOx generated from the equivalence ratio of 0.75 to 1.05 is reduced by 40%-45%. The mechanism of NOx generated by ammonia combustion was analyzed and discussed. It is found that NOx mainly comes from fuel. With the decrease of equivalent ratio, the concentration of O/H increases, the concentration of NHi decreases, and the proportion of side reaction that produce NO increases, which is more conducive to NOx generation. Discharge produces a large number of active free radicals, and the promotion of NO+NHi and NHi+NHi by NHi is an important reason for NOx reduction. Ammonia combustion assisted by plasma can significantly expand the combustion limit and reduce NOx, providing a new method for clean and efficient ammonia combustion.