Two units of the Chinshan nuclear plant are in the decommissioning process. However, the fuel assemblies residing in the core cannot be discharged, and it becomes an obstacle for the future decommissioning works. Even though the utility plans to construct a building containing all the dry casks, the utility still wants to remove the fuel in the core as soon as possible. Thus, the plant staffs have planned to use the New Fuel Storage Building (NFSB) which is an existing building as a temporary storage space for 14 dry casks. Because the original purpose of the NFSB is not for the spent fuel storage, related safety analyses should be conducted. This study has used the Computational Fluid Dynamics (CFD) methodology to analyze the temperature distribution of the NFSB space filled with the dry casks. The gateway of the NFSB is the main passage for air exchange with the atmosphere. Influence of different opening types on the gateway is also investigated in this study. Based on the results, the thermal safety for the NFSB containing 14 dry casks can be ensured by passive air cooling.