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Smoke exhaust is of vital importance for life safety and structural safety in a tunnel fire. The smoke movement pattern could affect smoke exhaust efficiency, so it is necessary to determine the patterns in shafts in tunnel fires with natural ventilation systems. However, previous studies have focused on this problem at standard atmospheric conditions, but the ambient pressure, which could have an effect on smoke movement characteristics and temperature distribution, decreases in high-altitude areas. First, theoretical analysis is carried out to find that the smoke velocity is higher under reduced pressure due to lower heat loss. In addition, a set of numerical simulations based on Fire Dynamics Simulator (FDS) is conducted to investigate the effects of ambient pressures on smoke movement patterns in vertical shafts in tunnel fires with natural ventilation systems. The results show that the critical Richard number deceases under reduced ambient pressure, and the higher smoke temperature and velocity caused by lower ambient pressure are the reasons for the decrease in critical Ri. We hope that our work can provide a design reference for tunnel natural ventilation system design in high-altitude areas.