Abstract: The GIS switch of a 1 000 kV substation had abnormal system current. Ultra-high frequency partial discharge, ultrasonic partial discharge, SF₆ decomposition test, X-ray detection and other technologies were used to conduct detection, and a high-precision simulation model was established to analyze and verify the fault. The results show that under the influence of ambient temperature change, the long busbar connecting pipe made of aluminum-magnesium alloy would produce a maximum axial displacement of about 47 mm when it was heated and expanded, and a maximum axial displacement of about 25 mm when it was cooled and contracted. During the displacement process, the long pipe would drive the short bus pipe with the same horizontal arrangement and a 90° angle, and apply a lever force to the vertical section of the switch. There was a rigid constraint on the fixed support frame at the bottom of the vertical section, and the pressure generated during the heating expansion was released. The tension and torsion force generated during the cooling contraction would directly act on the vertical section. The bolt without anti-loosening glue was loose under the action of tension and torsion force, resulting in the formation of local clearance on the joint surface, which in turn led to the abnormal current of the GIS system.