Abstract: This paper focuses on a case of abnormal resistive current identified during the live-line detection of a 110 kV metal-oxide lightning arrester. It conducted multi-stage tracking detection and comprehensive diagnostic analysis. By continuously tracking the live-line parameters such as resistive current and power factor angle, combined with off-line test data including infrared thermography, DC reference voltage (U1mA), and leakage current at 75% U1mA, moisture ingress inside the equipment were confirmed. Disassembly inspection revealed that the fixing holes on the top flange of the arrester were unsealed, allowing atmospheric moisture to penetrate. The study systematically describes the evolution of electrical characteristics associated with this defect, analyses the root cause of the sealing failure, and proposes a technical approach for early detection of moisture-related defects through coordinated on-line monitoring and infrared thermography. The findings provide practical guidance for condition-based maintenance and whole-process quality control of surge arresters.