The purpose of internal fire Probabilistic Safety Analysis (PSA) is to evaluate the core safety risk caused by internal fire in nuclear power plant, which has been widely used in the world. The plant is divided into a large number of fire compartments. In the preliminary analysis, it is usually conservatively assumed that all important component and cables in the fire compartment will be damaged by fire, and the resulting risk results may be overly conservative, which may lead to unrealistic risk insight. This paper mainly introduces the method of fire scenario analysis, and takes a switchgear room of Hualong nuclear power plant as an example to show the process of application, which is used to evaluate the fire risk. In this study, the fire simulation is performed by hand calculation and CFAST software to simulate the effects of a fire using the concept of Zone of Influence (ZOI). It also discusses High Energy Arcing Faults (HEAF) scenario and the effect of mechanical ventilation. This paper provides a practical reference for detailed fire scenario analysis in internal fire PSA.