Abstract: MFL(Magnetic Flux Leakage) inspection is one of the most commonly used methods for detecting corrosion defects in pipelines. Normally, an MFL system is divided into three parts: in-pipeinspector, out-of-pipelocating system and data analyzing system, and AGM(Above Ground Marker) is a crucial device to locate the corrosion. With the fact that most currently existing AGM devices lack flexibility and their data communication is cumbersome, a high-precision AGM system, which operates simply and automatically,was developed for MFL systems.According to the analysis of the specifications and functional goal of AGM, a piece of hardware was designed and developed, including recharging, power supplying, signal processing and controlling blocks. Corresponding software for timing and data acquisition was also developed, and verified by experiments. The results indicated that the signal of an in-pipe inspector, passing by the AGM at the level of 3 meters deep underground, could be effectively detected.