An inn fire occurring in the middle of the night usually causes a great deal more injuries and deaths. This article examines the case study of an inn fire accident that resulted in the most serious casualties in Taiwan's history. Data based on the official fire investigation report and NFPA921 regulations are used, and the fire scenes are reconstructed using the latest Fire Dynamics Simulator (FDS) program from NIST. The personnel evacuation time and time variants for various fire hazard factors of reconstructive analysis clarify the reason for such a high number of casualties. It reveals that the FDS program has come to play an essential role in fire investigation. The close comparison between simulation result and the actual fire scene also provides fire prevention engineers, a possible utilization of FDS to examine the effects of improved schemes for fire safety of buildings.