A simulation of the uninsulated duct firestop system was performed to review its application possibility to the building by analyzing the radiant heat transfer and its impact to the surrounding structures in the event of a fire. The simulation targets were divided by the presence or absence of an insulation material (1 m long) as floor and wall duct firestop systems, and the surface temperature, which is an important input data, was based on the existing verification test data. Then, the ambient air temperature, surrounding structure surface temperature, and radiant temperature at some distance from the duct were analyzed through simulations in the modeled virtual spaces. As a result of simulation, it is judged that the application of the uninsulated duct firestop system wrapped by the insulation material can significantly reduce the possibility of fire spreading due to the radiant heat.