Abstract: For problems that current antenna of through-the-earth communication system is too big, has poor mobility and cannot achieve bidirectional communication, a medium wave small square loop antenna with center frequency of 500 kHz was designed, and impedance characteristic, matching performance and communication distance of the antenna were theoretically analyzed and tested. The test results show that good impedance matching is achieved between the antenna and signal source, the matching network efficiency is approximately 1; operating bandwidth of the antenna is approximately 8 kHz, and antenna efficiency is 6.1×10-5, which are approximately equal to the theoretical results; the antenna has good radiation characteristic, the effective transmission distance is 30 m when input power is 0.25 W.