Abstract: With the increasing management requirements of substation fire protection, it is very important for the Substation personnel to master the secondary circuit knowledge of substation fire protection system. Take a 500 kV substation as an example, this paper analyzes the secondary circuit of fire fighting, including the control circuit of fixed fire extinguishing device and the signal circuit of fire protection system, it can provide a solution to improve the reliability of secondary circuit of substation fire protection system, and provide valuable reference for related personnel of substation fire protection.