Abstract:A 10KV power distribution line is in vicinity to the user in the electricity system, which has very lower insulation. Also the lower and medium voltage aerial lines usually have no efficient methods to protect from lightning. This article based on the reason why the 10KV insulated wire breaks down, using ANSYS to decide the lightning-proof clearance by analyzing the course of over-voltage discharging and its voltage rates distribution when lightning hits the circuit while the clearance cooperates with insulator. The clearance provided is only to arrest the lightning and does not serve any other purpose. But it priors to insulator to discharge when lightning hits so that the insulator can be well protected from being broken down or burnt, which ensures the line operates safely under over-voltage lightning.