National Synchrotron Radiation Research Center, Taiwan, is currently constructing the civil buildings and utility system of the Taiwan Photon Source. The air conditioning (AC) system of the storage ring tunnel was designed based on those of existing Taiwan Light Source and some international advanced accelerators. To predict the air temperature variation and flow pattern in tunnel more precisely, we applied 3D computational fluid dynamics scheme and built up a one-cell mock-up equipped with AC system. In the 3D numerical model, effects of magnets of the booster and the storage ring, girders, cable trays, front ends and the supplied air wind duct are all taken into account. The temperature variations and flow pattern were shown through the numerical simulation. Measured air temperature data in the one-cell mock-up were also collected.