It is a promising measure to transmit downhole information by acoustic wave through drill string. However, the transmission distance and quality of acoustic signals in field conditions are still not satisfied because the acoustic frequency spectrum in drill string is vulnerable to interference. This paper studied acoustic frequency spectrum characteristics in periodic drill string by experiments under different axial tensile forces. The multilevel method is established to evaluate frequency spectrum through drill string based on improved radar chart. The impact of axial tensile force on spectrum characteristics is analyzed by the proposed method. The results indicate that the axial tensile force has remarkable impact on the acoustic frequency spectrum and passband characteristics. The spectrum characteristics change with axial tensile force. These changes are reflected in the position, width, and attenuation of different passbands. And the extent of changes is distinct to different passbands. Both the evaluation indicators of each passband and spectrum are change with increasing axial tensile force. The spectrum quality is excellent in the range of 17‐134?MPa for axial tensile force in drill string. And when the force exceeds 152?MPa, methods such as adjusting the carrier frequency and relay installation location are necessary to improve the transmission quality of acoustic signals.
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