At present, most dust reduction methods do not consider the difference of dust particle diameter distribution along the tunnel and the applicability of droplet group under different parameters. In order to improve the dust reduction efficiency of each section, the particle size analyzer was used to analyze the dust samples of each section along the tunnel, and the migration law of dust of each particle diameter during the construction of Tunnel Boring Machine (TBM) was studied. The dust falling efficiency of droplets with different particle diameter under different sets of parameters was determined by the experimental platform built by ourselves. Research indicates: along the axis of tunnel, the farther away from the face of the tunnel, the higher the peak volume frequency of dust, and the distribution range of dust gradually narrowed to the direction of small particle diameter. The settling distance of dust with different particle diameter is quite different, and the content of small particle diameter dust shows an upward trend as a whole. The reduction efficiency of dust first increases and then decreases with the particle diameter, and there is particle diameter of the optimum dust reduction efficiency under the action of the same droplet group. The calculation formula of the air and water flow of the air-water nozzle and the particle diameter of the optimum dust reduction efficiency are put forward. This research can provide basis and guidance for multi-stage dust reduction and nozzle parameter setting in different sections of the tunnel.