Abstract: To clarify the migration law of pollutants under the limit distance of deadend pressin ventilation mode in an extralong highaltitude tunnel, the Milashan tunnel is taken as the research object to conduct migration law study on CO and dust under the limit distance of deadend pressin ventilation using the numerical simulation software Fluent. Moreover, the site monitoring data are analyzed. The results show the following: (1) The dust concentration on the tunneling surface increases first and then decreases with the ventilation time. The dust concentrates at the trolley close to tunneling face, and the dust concentration in the trolley area varies little during ventilation. (2) The dust concentration at the breathing surface of workers decreases step by step, whereas the CO concentration increases linearly to the peak, and then decreases in the form of a quadratic function. The time for CO concentration reaching the control standard is less than dust. (3) The CO concentration in the working area shows a quadratic function variation mode. The CO concentration lowers as the distance from tunneling face increases, whereas the dust increases first and then decreases to a stable value. Accordingly, the working time should be referenced from the time for dust concentration reaching the control standard. (4) The relationship between the dust diffusion distance and ventilation time shows cubic function, whereas the relationship between CO diffusion distance and ventilation time is linear.

Key words: highaltitude tunnel, extralong tunnel, deadend pressin ventilation, limit distance, pollutants, migration law