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Aiming at the problems of high manufacturing cost and easy damage of communication cable joint in the central cable type drill pipe, which is the cable transmission device for directional drilling in underground coal mine, an electromagnetic measurement while drilling（ＥＭ－ＭＷD） system used in coal mine is developed. The system is composed of special industrial computer, measuring tube, electromagnetic transmission tube, etc. It adopts the electromagnetic transmission mode with the drill pipe and the formation as the communication channel. The underground test results of coal mine show that the EM- MWD system works stably with correct measurement data and reliable electromagnetic transmission, and there is no need to use the central cable drill pipe, which not only improves the reliability of downhole directional drilling but also significantly reduces the drilling cost. The mine-used EM-MWD system could satisfy various requirements of directional drilling in coal mine.