The invention relates to a ranging and fault diagnosing device for an overhead power transmission line on the basis of magnetic field detection. The device comprises a three-dimensional magnetic field intensity measuring device arranged in an overhead power transmission line iron tower, and a display device arranged at the work end. The three-dimensional magnetic field intensity measuring device comprises a three-dimensional magnetic field sensor probe, a signal conditioning circuit, an A/D conversion module, a single-chip microcomputer, an FPRA logic processor and a radio frequency emission end. The display device comprises a displayer and a radio frequency receiving end connected with the displayer. The radio frequency receiving end and the radio frequency emission end transmit data through radio frequency. According to the ranging and fault diagnosing device, judging of short circuit faults in the overhead power transmission line and ranging of fault points can be achieved at the same time, the judgment result is good, and the distance is measured accurately.