Abstract: Due to the complicated structure and high accuracy of the main nozzle, its air field characteristics can not be tested directly, so the numerical simulation is applied in this paper. The distribution of the air flow speed along axial and radial direction in the main nozzle is investigated, and flow fields under various supply pressures are also studied. The results indicate that the flow field in the main nozzle is very complicated, including the supersonic area and the backflow phenomenon, and if the supply pressure is too high, the backflow phenomenon will influence the weft insertion directly, so the supply pressure should be ensured at the range of 0.2-0.4Mpa. This study is helpful to the optimization of air jet loom weaving process.