Abstract: At present, grounding fault protection of most flexible grounding system directly adopts the zero-sequence time-limit overcurrent protection of traditional low-resistance grounding system, which has problems including insufficient protection sensitivity and insufficient utilization of fault information when grounding via high-resistance. When single-phase grounding fault occurs in flexible grounding system, the characteristics of zero-sequence admittance measurement of healthy lines, upstream and downstream line of the fault point before and after switch-in of parallel small resistance are analyzed. It is found that the zerosequence admittance measurement of healthy lines and the downstream line of the fault point remain unchanged, and the modulus ratio of admittance before and after switch-in of parallel resistance is 1, while the ratio of the upstream line of the fault point is much greater than 1. Based on this, a discrimination algorithm of grounding fault direction based on variation of zero-sequence admittance in flexible grounding system is proposed. Digital simulation and field experiment data show that this method has high accuracy, and it will not be easily affected by factors such as line parameters and transition resistance. The proposed method can be used in conjunction with zero-sequence time-limit overcurrent protection, which can meet practical field requirements.