To improve the drawbacks of original designed air-assisted duct of boom sprayer, such as huge wind consumption, low efficiency, large coefficient variation (CV) of wind velocity, 3-D flow field of interior duct and exterior free flow region was simulated by using computational fluid dynamics (CFD). Flow field analysis of the simulation results showed that there were strong swirling and narrow flow channel in the flow field of prototype air-assisted duct. It also resulted in large CV in exterior free flow field. In response to these shortcomings, two kinds of improvement designs were proposed. Firstly, the outlet size and outlets spacing was decreased to greatly reduce the CV of wind velocity, but could not fundamentally change the flow structure. Secondly, on the basis of above optimization, a flow deflector was added to overcome the strong swirling flow and narrow flow channel. Prototype testing indicated that final scheme achieved high efficiency and small wind speed variations.