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Taking into account the structural characteristic of the high voltage overhead transmission line,an electromagnetic testing system based on the high speed ARM microprocessor STM32F407 is designed in this paper,which combines with eddy current testing and leakage magnetic flux testing systems. In the paper we introduced the Direct Digital Synthesis (DDS) based eddy current excitation signals generator module,eddy current signals extraction module which was realized by lock-in amplifier,leakage magnetic detection module which employed differential measurement technique,and A/D conversion module etc. In addition we also designed the host computer programs with LabVIEW,and it communicated with the system via the USB bus. The whole system possesses advantages such as strong real-time,easy to integration and remodeling. The validity of the proposed system is proved by a real detection process,which is implemented to find out the defects in butt joint and aluminum strands on power transmission line.