Abstract: Aiming at the abnormal increase of the dielectric loss of the grid-side winding to the valve-side winding and to the ground of a pole I high-end 600 kV converter transformer, established the distributed capacitance equivalent model between the internal components of the converter transformer. Meanwhile based on the analysis of the T-type network model for the abnormal dielectric loss of the converter transformer, the dielectric loss isolation test of the core and the clamp, and the decoupling calculation of the related dielectric loss and capacitance of the grid-side and valve-side windings, the qualitative analysis and quantitative analysis of the causes of the excessive dielectric loss of the grid-side winding of the converter transformer are completed. With the help of body heating treatment and frequency domain dielectric spectrum detection test, it is confirmed that the abnormal dielectric loss is caused by the damp deterioration and insulation degradation of the converter transformer grid-side winding to the core.