As an important reactive power compensation device in a power system, the high voltage shunt reactor has the function of compensating for the capacitance effect of a transmission line, regulating reactive power and voltage. A reactor fault in operation will pose a serious threat to the safety and stability of the power grid, and so it is of great significance to analyze the cause of a fault for the maintenance of power grid security. A fault on a 500 kV oil-immersed high voltage shunt reactor is analyzed based on the component analysis of characteristic gas in the oil chromatogram, the calculation of gas content ratio, the trend analysis of characteristic gas and the characteristic gas calculation of the device before return to the factory. Diagnostic tests of DC resistance, insulation and dielectric loss, vibration, noise and temperature rise etc. are carried out for further investigation, and it is concluded that the cause of the accident is poor insulation between silicon steel sheets of reactor core, this being achieved by employing winding inspection, core inspection, connection part inspection, sample detection and analysis. Finally, corresponding preventive measures are presented, providing reference for process control, fault diagnosis and maintenance of the same type of high voltage shunt reactor. This work is supported by the National Natural Science Foundation of China (No. 51807028).