Fires in underground communication tunnel lead to a major disaster in modern society. Combustible toxic gases are generated within long and narrow space, which block the oxygen supply and expand combustion rapidly. It’s difficult for the firefighter to enter the tunnel. In order to prevent the recurrence of such a disaster, an accurate determination of the fire origin, an analysis of the causes of the fire, and a corresponding preventive measure should be established. However, the burned out communication cable tunnel due to its complicated structure and difficulty in extinguishing is which fire patterns are destroyed or contaminated, making it difficult to determine the point of fire origin. In this study, KT Ahyeon Branch's communication fire case is analyzed. In burned communication tunnel where it is difficult to determine the point of origin through fire pattern, it was confirmed that fire spread verification using structural environmental conditions and simulation (FDS) can be the main basis for fire origin determination.