Abstract: In order to achieve the accurate detection of small structures in the working face, the relative transmission coefficient tomography technology related to in-seam wave is studied. According to the management and analysis of the data collected at the site of some projects, and the comparison between the CT imageing technology and the actual geology data, the relative transmission coefficient tomography technology is proved to be able to show the location and the form of a roadway shorter than 1/3 thickness of coal seam clearly and accurately. Through field engineering test, the stability of the relative transmission coefficient tomography technique is satisfying, and the accuracy is quite high. This can serve as an very effective method to detect the internal small structures in the working face.