Abstract: Abstract: The pesticide solutions include both fat soluble and water soluble. The permeability of fat soluble pesticides is greater than that of water soluble pesticides. So its infiltration effect is better than that of water soluble pesticides. It is necessary to break up the oil into a fine oil bead by the intense mechanical agitation when fat soluble pesticide mixed with water. However, the emulsion oil droplet dispersion degree is not high. During the storage period, it is very easy to produce oil beads and break the emulsion, and after that, it's hard to recover. Inline mixed pesticides can reduce the harm of pesticides to operators, the environment, and non-target organisms. At present, the mixed pesticide is mainly mixed with water soluble pesticides. It is difficult to study the inline mixing of fat soluble pesticides due to their insolubility in water. The purpose of this study was to determine the effective length of the mixer under the condition of uniform mixing of fat soluble pesticide (rapeseed oil). Comparative analysis was made for the different diffusion angle, different injection methods, different contraction angle, and different mixing tube length. The main components of swirling jet mixer were shrink tube and diffuser. When the shrinkage and diffusivity were determined, the influencing factors of the effective length of the mixer were the contraction angle, the length of the mixing tube and the diffusion angle of the diffuser. Structure parameters of the mixer were optimized by orthogonal experiments with three factors and three levels, including the contraction angles of 19°, 22° and 25°, the mixing tube length of 20, 12 and 8 mm, and the diffusion angle of 10°, 14° and 18°. To confirm the content of the study, 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. According to different physical and chemical properties of pesticides, rapeseed oil was used as substitute for fat soluble pesticide, and it was added with oily fluorescent agent LUYOR-6100. The orthogonal experiment results showed that the probabilities of the three factors were 0.206, 0.004, and 0.025, respectively. The mixing tube length of swirling jet mixer for mixing uniformity was the most significant, followed by diffusion tube diffusion angle, no significant effect was the contraction angle shrink tube. The order of the three levels of the contraction angle of the contraction tube on mixing uniformity was 25°, 19°, and 22°. The order of the three levels of the mixing tube length on mixing uniformity was 20 mm, 12 mm, and 8 mm. The order of the three levels of the diffusion tube diffusion angle on mixing uniformity was 18°, 14°, and 10°. On the premise of fat soluble pesticide mixing, the effective length of the swirling jet mixer of 25°, 20 mm, and 18° was relatively short. The mix combination of 25°, 20 mm, and 18° was better because its effective length was shorter in the realization of fat soluble pesticide (rapeseed oil) uniform mixing. Its effective length is 69 mm, the contraction angle is 25°, the mixing tube length is 20 mm and the diffusion angle is 18°. In conclusion, swirling jet mixing device 252018 (25°, 20 mm, and 18°) can be used as a shorter mixing device structure in the experiments of studying the adaptability of the sprayer and nozzle structures.