Fire safety is an important part of nuclear safety. Based on the safety and economic requirements of nuclear power plants, combined with domestic and foreign fire regulations and standards, Pyrosim is used for the first time to conduct full-scale 1:1 numerical simulation based on a nuclear power project to study the effects of the setting principle, system selection and design parameters of the nuclear island cable fire fighting system on the fire extinguishing performance. The optimal scheme is determined by analyzing the heat release rate, ambient temperature and flue gas height. In addition, by building a test bench, the electrical insulation performance of a certain type of sprinkler used to protect the cable is studied to reduce the risk of electric shock and electrical failure caused by water-based fire extinguishing system. The results show that if the fire load density is greater than 400 MJ/m2 as the basis of whether to set automatic fire extinguishing system in the cable area, the open fire extinguishing system is preferred, and the sprinkler intensity is 10 L/(min m2); The leakage current value of the nozzle is less than 0.15 mA, which is safe for human body and electrical equipment, and can be used to protect the medium-voltage cable of the nuclear island.