The invention relates to a device and method for locating the fault of series connection of alternating current into a direct-current system. The voltage of the positive pole of a direct-current busbar, the voltage of the negative pole of the direct-current busbar, the current of the positive pole and negative pole of each branch are acquired through sensors; the fundamental frequency alternating-current components of the voltage of the busbar and the fundamental frequency current components of each direct-current branch are extracted; and analysis and comparison are carried out on the amplitude and phases of alternating-current voltage and alternating current, so that a branch into which alternating current is connected in series can be determined. With the method provided by the invention adopted, the problem of incapability of a traditional direct current line selection device to accurately locate a fault branch in a fault according to which alternating current is connected in series into a direct-current system can be solved. The device of the invention has a high practical value.