This paper improves the fire detection method for distribution systems based on RBI theory, establishes a reliability model for fire risk in distribution systems, and uses the risk assessment results to formulate a fire detection plan suitable for the evaluation object. The probability of a fire accident occurring in a distribution system line is expressed using five parameters. Based on the classification standards for failure probability levels, the probability level of a fire occurring in a distribution system line is determined. The severity of fire consequences in distribution system lines is summarized using six parameters. Based on the classification criteria for consequence levels, the consequence level of fire accidents in distribution system lines is determined. Finally, the fire risk reliability of the distribution system is assessed using the fire risk matrix. Taking a certain substation as an example, an RBI assessment of its fire risk was conducted, identifying three high-risk projects. Corresponding fire prevention and control measures were proposed for rectification.