Abstract: The working cylinder of the 500 kV circuit breaker operating mechanism in a certain 500 kV substation malfunctioned. The reasons for the leakage of the working cylinder were analyzed using macroscopic observation, chemical composition analysis, mechanical performance testing, microscopic analysis. The results show that the working cylinder experienced excessive temperature during the heat treatment process, resulting in coarse grain size of the cylinder body material, intergranular segregation, and the formation of remelted spheres at grain boundaries, leading to a decrease in grain boundary strength. During long-term operation, cracks were generated along the grain boundaries of the working cylinder under the action of oil pressure, and the cracks further propagated, ultimately leading to leakage of the working cylinder. The research results had certain guiding significance for the fault analysis of circuit breaker mechanisms and the formulation of heat treatment processes for working cylinders.