The invention proposes a fault alarm of a boundary switch on a power distribution line post. The fault alarm comprises a shell, a balancing ball mechanism and an alarm ball mechanism, wherein the balancing ball mechanism is arranged in the shell and comprises a spring, a balancing ball and a balancing ball box, the alarm ball mechanism comprises a balancing ball sliding groove, a linkage rod, an alarm ball positioning rod, an alarm ball and an alarm ball support groove, the alarm ball is connected with a connection rope, a top end of the linkage rod is arranged below the alarm ball positioning rod, a horizontal top rod is arranged at a top end of the linkage rod, and when the balancing ball touches the linkage rod, the horizontal top rod of the linkage rod is used for ejecting the alarm ball positioning rod and the alarm ball slips off the alarm ball support groove. By the fault alarm, an alarm signal can be given out according to explosion vibration of the boundary switch on the post, and an operation maintainer is easy to find a fault signal so as to judge which post does a boundary switch fault come from.