Abstract: In view of problem that a 330 kV start-up standby transformer took place relay protection action in no-load charging, the paper indicated that the action was caused by grounding fault of A-phase current transformer through analyzing charging current waveform recorded by electric quantity analyzer, and gave several suggestions to improve safety and reliability of charging test of large-scale transformer: strengthen insulation inspection for current and voltage transformers of high-voltage side of transformer; properly increase sensitivity of main and backup protection to prevent relay protection action by inrush current; it should record for three-phase current and voltage of high-voltage side of transformer in charging process.