Abstract: As for the abnormality of secondary voltage of 500kV capacitor voltage transformer (CVT) in operation, it is checked clearly by way of site inspection and disassembly at factory that the cause for such abnormality is due to the protection arrester with both ends of the compensation reactor in CVT fault and insulation strength reduction of arrester. The reasons that the secondary voltage value of CVT is small and the phase lead in CVT are qualitatively analyzed by the phasor. Combined with the test data of the equipment, the secondary output voltage of fault CVT with load is quantitatively analyzed.