With the rapid development of smart grid technology in China, various fault location technologies for transmission lines have emerged continuously. The non-contact fault location device based on magnetic field measurement has good application prospects due to its convenient installation, high efficiency, accuracy and safety. In order to study the magnetic field characteristics of such devices under different working conditions of transmission lines, this paper establishes a finite element model for calculating the magnetic field environment of non-contact fault location devices used in 220kV transmission line tower-line systems. The normal operation conditions and typical fault conditions are calculated to determine the magnetic field strength range and waveform characteristics under different operation conditions. The results show that the measurement of spatial magnetic field signals can effectively reflect the operation status of transmission lines, so as to achieve precise location of fault points in transmission lines. The research results of this paper provide technical support for the application of non-contact fault location devices based on magnetic field measurement.