In this study, the radon concentration in the indoor and underground water of Gwangju subway stations were measured from December 2021 to September 2022 with long-term(more than 90 days) and short-term cycles(48hr). As the measurement places, platform, office, and pump room were selected. Ventilation time and underground water volume are main influencing factors for indoor radon concentration. As a result of study, the long-term radon concentration of indoor air was 17.3 ㏃/㎥ at the platform, 31.0 ㏃/㎥ at the office, and 112.1 ㏃/㎥ at the pump room. The radon concentration per hour in indoor air was measured using Rad-7 with short-term cycle(48 hours), the highest concentration was measured at 53.9 ㏃/㎥ at the platform, 88.9 ㏃/㎥ at the office, and the radon concentration in the office showed about 39% higher than that in the platform. The radon concentration of indoor air in the subway stations was affected by bedrock and depth. As a result of additional measurement for a week in the pump room with a high concentration of radon distribution, it was confirmed that the radon concentration decreased periodically at the operating time of the ventilation system and decreased by 86%. The three sites with a large amount of underground water showed high indoor radon concentration in order....