Abstract: Abstract: The on-line pesticide mixing is a process with higher productivity and safer operating conditions, which can reduce pesticide wastes and pollution. To characterize jet mixing characteristics for on-line injection of pesticides, a new mixing device based on the swirling jet mechanism was conceived and developed to improve the mixing uniformity of water and fat-soluble pesticides. The swirl jet mixer consisted of a spiral curved shrink tube, a diffuser and a spiral flow generator, and could accelerate the two-phase flow's spiral movement and blend the 2 kinds of liquids. In order to increase the swirling efficiency of swirling jet mixer, the spiral flow generator had 3 guide vanes (height was 6.5 mm, wrap angle was 15°); the diffuser had also 3 guide vanes, but its height was from 0 to 6.5 mm; the pitch of the spiral curved shrink tube was 128 mm, and its converging angle was 16°. A computational fluid dynamics program (FLUENT) was used to simulate the flow field inside the swirling jet mixer to optimize its design. The boundary conditions of the water and the rapeseed oil were defined to a pressure input. The boundary condition of mixer outlet was defined to a pressure output. The total gauge pressure, the initial gauge pressure and the turbulence intensity were defined. The solid wall was supposed to be a non-slip, impervious and adiabatic boundary. The segregated and implicit modeling was identified by incompressible fluid. The steady flow and the k-??modeling of turbulent flow were identified. The mixing uniformity was evaluated by introducing the uniformity index, and the 2 kinds of liquids were blended homogenously as the uniformity index was 1. Simulation results showed that the uniformity index was 0.999 5 across the entire cross sections inside the mixer, confirming the uniform mixture characteristics with the new mixer design. To verify the simulation results, the experiments were conducted in which the fluorescent tracers were mixed with the pesticides to enable the camera to track the mixing process in the transparent chamber. The mixing process in the on-line injection system actually is a process that water from the water tank and pesticide from the pesticide tank are mixed in the mixer through the liquid convection and pervasion. Water was set as the main flow phase, with the density of 1.0 g/cm3, the kinematic viscosity of 1 mm2/s and the surface tension of 72 mN/m. The rapeseed oil was set as the second flow phase, with the density of 0.9 g/cm3, the kinematic viscosity of 43 mm2/s and the surface tension of 29.8 mN/m. The mixing experimental system for the mixer included water intake system, pesticide intake system and image acquisition system. The water intake system contained water tank, water pump, water flowmeter, water manometer and water piezometer, while the pesticide intake system contained pesticide tank, pesticide pump, pesticide flowmeter, pesticide manometer and pesticide piezometer. The water and the pesticide were mixed straightway by the pesticide mixer before they were sprayed by a nozzle. The experimental results showed that the pressures of the water piezometer, the pesticide piezometer and the outlet of the mixer were separately 0.23, 0.22 and 0.22 MPa. The maximum mixing ratio was 99.442 5% at the outlet of the mixer, so the mixer had significant uniformity index for the fat-soluble pesticide. The simulations and the verification tests indicated that the mixing uniformity was very good in the swirling jet mixer when applying fat-soluble pesticides through on-line injection spray. Therefore, the new swirling jet mixer would solve the non-uniform mixture problem associated with conventional mixers and could significantly improve the on-line injection technology to reduce pesticide waste. The swirling jet mixer was desirable, and its structure was compact. Furthermore, there were mandatory and optional items, and we could choose what we needed according to the spraying conditions. It is conducive to the application and marketing promotion of the plant protection machineries.