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Currently, many airports in China are being built or retrofitted. Reducing energy consumption in airport terminals is of the utmost urgency. This paper describes on-site measurements of indoor thermal environment and air infiltration of a hub airport in winter in southwest China. Air velocity measurements with air volume balance check and thermal balance check were applied to assess air infiltration rates in terminal buildings. In unsecured halls, air infiltration rates were 0.61 air change per hour (ACH) (6.6 m3/(h m2)) and 0.28 ACH (3.0 m3/(h m2)) when space heating was on and off, respectively; while in secured piers, those two air infiltration rates were 0.42 ACH (2.6 m3/(h m2)) and 0.24 ACH (1.5 m3/(h m2)). Air infiltration consumed 66–92% of heat supplied by space heating systems, showing that winter air infiltration significantly affects indoor thermal environment and energy consumption in terminal buildings where air flows out through the doors of service walkways and open skylights on the roof. Furthermore, influences of building characteristics, space heating systems and outdoor temperatures on winter air infiltration in large space buildings were analysed. This research helps to clarify the key issues influencing indoor thermal environment and proposes solutions for energy saving in terminal buildings