Aimingat a 220kV line trip fault in a substation, by reviewing the weather process at the time of the fault occurrence and combining with the on-site troubleshooting situation, it was analyzed that extreme weather caused the lead to wind deflection flashover on the framework crossbeam, which is the main cause of this fault. The cause of the fault was further confirmed through the calculation of the phase to ground distance between the lead wire and the framework crossbeam. Measures such as reducing lead sag and adding post insulators to pull the lead have been proposed to prevent wind deflection faults in the lead.