In the construction of mountain expressways, condensation on the cement concrete base layer of tunnels is a common issue that can significantly compromise the bonding conditions between pavement layers. Field investigations and theoretical analyses were carried out in four tunnels of an expressway under construction. The condensation mechanism was examined using calculation methods prescribed by meteorological observation guidelines, and the influence of various factors on condensation was evaluated. Effective mitigation measures were then identified. The results show that relative humidity, dew point temperature, and tunnel alignment exert primary control over condensation. Specifically, when the internal-external humidity differential exceeds 20% and a high proportion of circular curves is present at tunnel portals, severe condensation occurs. In contrast, wind speed and air pressure were found to have negligible effects. To alleviate condensation, measures such as heating, chemical dehumidification, and enhanced ventilation are recommended.