In this study, the internal environment such as the air temperature, humidity, and wall temperature of the underground utility tunnel, was analyzed. The current status and problems of the air conditioning system were examined by analyzing the capacity of the exhaust fan and the air velocity inside the utility tunnel. The field experiment showed that the utility tunnel has a relative humidity of 95% or higher for most sections during summer. The deviation of the internal air temperature was about 4 ℃ depending on the section, and the dew condensation occurred. However, most of the exhaust fans has a capacity below the standard minimum air velocity of 2.5m·s-1. In particular, in the section where dew condensation occurred, the air velocity was 0.26 to 0.97 m·s-1, indicating the presence of stagnant air inside the facility. Therefore, this study attempted to minimize dew condensation by calculating the proper exhaust fan capacity using computational fluid dynamics and installing circulation fans and duct systems in the section where the dew condensation occurred. As a result, when a circulation fan was installed, it was possible to increase the air velocity inside the utility tunnel, and the relative humidity could be reduced by about 78%. By installing a duct, the direct supply of external air or the discharge of internal humid air was simulated for the section where dew condensation occurred. The result showed that the relative humidity could be reduced by about 78% when the duct system was operated in the intake direction.