Abstract: In response to an operational anomaly of the 110 kV high-voltage bushing of a main transformer in a substation, infrared precise thermometry revealed oil temperature stratification, and off-line tests showed that dielectric loss and capacitance exceeded the standard limits. Disassembly inspection confirmed that the root cause was a significant reduction in internal oil volume. Mechanism analysis indicated that the micro-negative pressure generated by the corrugated oil conservator continuously sucked insulating oil through the aged and poorly sealed gasket, leading to a drop in oil level. Additionally, the inclined arrangement of the conservator caused residual oil retention, resulting in distorted oil level indication. This created a concealed defect that was difficult to detect during routine inspections. Based on these findings, a targeted hazard management plan was formulated, emphasizing routine infrared precise temperature measurement inspections, periodic sampling inspection of sealing components, and special inspections of same?batch equipment, providing engineering reference for the prevention of latent faults in similar main transformer bushings at 110 kV and above.