Abstract: High-voltage cables have become the primary power transmission mode of power grids, and their service reliability constrains the safe and stable operation of power systems. Owing to the concealed nature of cable lines, complex operating environments, and heavy load pressures, traditional inspection methods could hardly satisfy the requirements of fault early warning and condition assessment. Therefore, current status of cable faults in Beijing area was analyzed in the paper. Taking the fault analysis of 220 kV cable intermediate joints as an example, a multi-dimensional collaborative detection system for high-voltage cables was constructed, and the role of online partial discharge (PD) monitoring technology for insulation defects early warning in high-voltage cables was analyzed. First, power-frequency withstand voltage tests and PD tests were performed on cables with abnormal PD signals identified during retesting, through which the fault source was located inside the intermediate joint. Subsequently, preliminary localization of the defect was achieved using an oscilloscope based on echo reflection. Through fault dissection analysis, the defect position was determined to be inside the epoxy bushing of the cable intermediate joint. The internal structure of the epoxy bushing was then examined by computed tomography, and the PD anomaly was revealed to be caused by an air gap. Finally, electric field simulations were conducted under two conditions of normal operation and the presence of an air gap, by which the correlation between the air gap defect and the PD anomaly was verified. The study could provide technical support for fault early warning, condition assessment, and operation & maintenance strategies of high-voltage cables, and offer both application value for engineering practice and useful guidance for similar implementations.