The invention discloses a fault location method in the case of over-zone power supply by adopting a railway AT power supply mode. The method is characterized by comprising the following steps: according to an over-zone AT section position where fault happens, bus voltage U of an AT station at the fault happening over zone is acquired, a downlink feeder T line current It1, a downlink feeder F line current If1, an uplink feeder T line current It2, and an uplink feeder F line current If2 of a former substation of the substation at the fault happening over zone are acquired, and then the fault location L as described in the specifications is calculated according to the following formula which is described in the specifications, wherein x is calculation reactance, Di is the length of each AT section, and X0 is unit reactance. When the method of the invention is applied, the fault distance at the over-zone position can be rapidly and accurately determined, and the fault clearing efficiency of the over zone can be enhanced.