Abstract: Objective: Jinan urban rail transit power supply system operates by main substation power supply. The power cables in the system are relatively long, cable grounding capacitance and main transformer leakage inductance may match harmonic parameters, and amplify background harmonics within the power supply system and cause resonance. Therefore, it is necessary to analyze the causes of harmonic pollution in the power supply system and propose improvement measures. Method: Taking the case of main transformer switch protection action tripping fault in Jinan Metro Line 1 power supply system, the principles and characteristics of harmonic resonance are expounded.Result & Conclusion: The improvement measures obtained from analyzing the main transformer switch protection action tripping fault include differential protection technology, transformer control reinforcement, and equipment inspection enhancement.