Based on the detection and analysis of the abnormal phenomenon in UHV transformer partial discharge test, this paper eliminates the influence of external disturbance and the test system, analyzes the structure characteristics of transformer magnetic shielding and grounding, and carries out the discharge location. It first discovers the fracture of magnetic shielding ground line in UHV transformer, which causes the magnetic shielding floating potential discharge and thereby produce a large amount of acetylene gas. The defect is eliminated successfully, and then the transformer operates normally by replacing the magnetic shielding ground line. The analysis shows that the integrated use of partial discharge test, ultrasonic positioning method and oil chromatographic analysis can accurately diagnose defect types and position them. This work is supported by National High-tech R & D Program of China (No. 2015AA050204).