The invention relates to an overhead transmission line distance measurement and fault diagnosis method based on magnetic field detection. The method includes the steps that the nth current element d1 on a transmission line and a magnetic field generated by the mirror image d1' of the current element d1 on a certain point in the space are acquired; the catenary equation of the transmission line is calculated; the magnetic field acquired in the first step is substituted into the catenary equation of the transmission line, and the magnetic field intensity generated by N phases of wire on the spatial point P can be calculated based on the superposition principle, wherein each phase of wire is in (2K+1) span length; the current In and the magnetic field intensity Bn are measured, and the distance h is calculated. Besides, if the magnetic field direction measured by a certain three-dimensional magnetic field intensity measurement device suddenly changes at a certain moment, it is shown that the transmission line where the magnetic field intensity measurement device is located has a short-circuit fault. Through the method, judgment on the short-circuit fault in the overhead transmission line and distance measurement on a fault point can be performed at the same time, the judgment effect is good, and the measured distance is accurate.