Abstract: To investigate the spatial distribution patterns of dust concentration and particle size during the excavation phase of large cross-section tunnels using the bench tunnelling method with a cantilever roadheader, this study conducted field measurements based on the construction of a double-track high-speed railway tunnel in Southwest China. The dust concentration and particle size distribution in various tunnel zones were measured, the dust diffusion patterns under forced ventilation were summarized, and the longitudinal zonal characteristics of dust diffusion were elucidated. The field measurement results indicate: (1) The distribution characteristics of dust concentration in different areas of large section bench tunnel are different, and the concentration of dust in the turbulent zone (0 ~ 30 m) fluctuates significantly (peak 1424 mg/m³), the concentration in the settlement zone (30 ~ 100 m) decreases rapidly from 960 mg/m³ to 288 mg/m³, and the stable zone (> 100 m) is less than 70 mg/m³. The large particle replenishment mechanism of the lower step makes the settlement area larger than that of the small section tunnel. There are vortex, return air and diffusion areas in the upper step, and the dust concentration in the return air area changes non-monotonically. (2) The particle size of dust produced under different tunnelling conditions is significantly different: the oblique advance is the largest (D₅₀ = 15.59 μm ), the left and right swing milling is the smallest (D₅₀ = 8.52 μm), and the proportion of respirable dust is 22.68 % ~ 41.22 %. (3) The dust particle size gradually decreases with increasing distance from the tunnel face: large dust particles predominantly deposit within 25 m behind the face, while the respirable dust fraction exceeds 80% beyond 100 m. At 300 m behind the excavation face, dust settlement becomes relatively uniform, and the average dust particle size is relatively small, ranging between 6.02 µm and 7.11 µm.